Graph-based current transformer saturation identification method, system, device, and medium

Abstract

The application relates to the technical field of current transformer saturation identification, and discloses a current transformer saturation identification method, system, equipment and medium based on graphics, which comprises the following steps: sampling the secondary side current of the current transformer after a fault occurs; generating a first geometric figure according to two continuous extreme points of the secondary side current and the midpoint of the sampling points between the extreme points; judging whether the first geometric figure satisfies a first criterion, if not, considering that the current transformer is saturated; if yes, generating a second geometric figure and a third geometric figure according to three continuous extreme points of the secondary side current and the midpoint of the sampling points between adjacent extreme points; judging whether the second geometric figure and the third geometric figure are similar according to a second criterion, if yes, considering that the current transformer is not saturated, and if not, considering that the current transformer is saturated. The application guarantees the speed of differential protection and the reliability of the criterion through double criteria, and realizes continuous and reliable monitoring of the whole process of CT saturation.

Description

Technical Field

[0001] This invention relates to the field of current transformer saturation identification technology, and in particular to a graph-based current transformer saturation identification method, system, device, and medium. Background Technology

[0002] As a core component of the power grid, the operational reliability of transformers directly determines the safe and stable operation of the entire power system. In transformer protection configurations, ratio-controlled differential current protection is widely used as the main protection due to its sensitive identification capability for faults within the protection zone. However, the voltage levels on both sides of the transformer differ, resulting in differences in the capacity, turns ratio, and rated secondary load of the configured current transformers (CTs). When an external fault occurs, the transient saturation levels of the CTs on each side differ, leading to a significant increase in unbalanced current. Especially when one side's CT is severely saturated while the others are transmitting normally, a large false differential current will be generated, easily triggering maloperation of the differential protection and posing a serious threat to the safe operation of the power grid.

[0003] Currently, the second harmonic blocking principle and the time difference method are commonly used in engineering to prevent maloperation of protection caused by CT saturation. The second harmonic blocking principle is based on the characteristic that the second harmonic content in the saturation current is high, and the blocking is achieved by detecting the harmonic components. The time difference method uses the time difference between the occurrence of the fault and CT saturation to distinguish whether the CT saturation is caused by an internal or external fault. However, both methods have certain limitations. The second harmonic detection method requires precise extraction of the decrease amplitude of the second harmonic within half a cycle. However, the content of the second harmonic in the early stage of current transformer saturation may not be significant, and its trend is difficult to calculate accurately. Furthermore, when a serious fault occurs, the CT operates completely above the saturation inflection point, with very low harmonic content, which is completely different from the electrical characteristics of conventional CT transient saturation. This makes it impossible for the conventional second harmonic method to reliably block differential protection. On the other hand, the time difference method requires precise location of the fault occurrence time and the differential current occurrence time. When a serious fault occurs, the time difference is very short, less than 1 / 4 cycle, and the protection may fail. Moreover, the time difference method adopts a one-time detection mechanism. Once the first detection is missed, subsequent monitoring of the CT transient saturation process cannot be achieved, further weakening the reliability of the protection. Summary of the Invention

[0004] To address the aforementioned technical problems, this invention provides a graphical current transformer saturation identification method, system, device, and medium. By analyzing the degree of distortion of local geometric features of the waveform, accurate and efficient current transformer saturation identification is achieved.

[0005] In a first aspect, the present invention provides a graphical method for identifying saturation in current transformers, the method comprising: The secondary current of the current transformer after the fault occurs is sampled, and a first geometric figure is generated based on two consecutive extreme points of the secondary current and the midpoint of the sampling point between the two consecutive extreme points. Determine whether the first geometric figure satisfies the preset first criterion. If it does not satisfy the criterion, the current transformer is considered to be saturated. If satisfied, then based on the three consecutive extreme points of the secondary side current and the midpoint of the sampling points between adjacent extreme points, the second and third geometric figures are generated respectively. According to the preset second criterion, it is determined whether the second geometric figure and the third geometric figure are similar. If they are similar, the current transformer is considered to be unsaturated; otherwise, the current transformer is considered to be saturated.

[0006] Further, the step of generating the first geometric figure based on two consecutive extreme points of the secondary current and the midpoint of the sampling point between the two consecutive extreme points includes: The two consecutive extreme points of the secondary current and the midpoint of the sampling point between the two consecutive extreme points are respectively taken as the first extreme point, the second extreme point and the first midpoint; Connect the first extreme point, the first midpoint, and the second extreme point sequentially end to end to generate the first geometric shape.

[0007] Furthermore, the step of determining whether the first geometric shape satisfies a preset first criterion, and considering the current transformer to be saturated if it does not, includes: The line connecting the first extreme point and the first midpoint is taken as the first line, the line connecting the second extreme point and the first midpoint is taken as the second line, and the line connecting the first extreme point and the second extreme point is taken as the third line. The ratio of the length of the first connecting line to the length of the second connecting line is used as the first indicator, and the ratio of the first indicator to the length of the third connecting line is used as the second indicator. Determine whether the first indicator meets the threshold range of the first indicator and whether the second indicator meets the threshold range of the second indicator. If not, the current transformer is considered to be saturated.

[0008] Further, the step of generating the second and third geometric figures based on the three consecutive extreme points of the secondary current and the midpoint of the sampling points between adjacent extreme points includes: The three consecutive extreme points of the secondary current are respectively designated as the third extreme point, the fourth extreme point, and the fifth extreme point. The midpoint of the sampling points between the third extreme point and the fourth extreme point is designated as the second midpoint, and the midpoint of the sampling points between the fourth extreme point and the fifth extreme point is designated as the third midpoint. Connect the third, fourth, and fifth extreme points one after the other in sequence to generate a second geometric figure; Connect the second midpoint, the fourth extreme point, and the third midpoint sequentially end to end to generate the third geometric figure.

[0009] Further, the step of determining whether the second geometric figure and the third geometric figure are similar according to a preset second criterion, and if so, considering the current transformer to be unsaturated, and otherwise considering the current transformer to be saturated, includes: Based on the corresponding vertices between the second geometric figure and the third geometric figure, determine the corresponding angles and corresponding sides between the second geometric figure and the third geometric figure; Determine whether the difference between the corresponding angles of the second geometric figure and the corresponding angles of the third geometric figure meets the angle difference threshold range, and whether the length ratio between the corresponding side of the second geometric figure and the corresponding side of the third geometric figure meets the side ratio threshold range. If the conditions are met, the second geometric figure and the third geometric figure are considered similar, and the current transformer is not saturated; otherwise, the second geometric figure and the third geometric figure are considered dissimilar, and the current transformer is saturated.

[0010] Secondly, the present invention provides a graph-based current transformer saturation identification system, the system comprising: The first criterion module is used to sample the secondary current of the current transformer after the fault occurs, and generate a first geometric figure based on two consecutive extreme points of the secondary current and the midpoint of the sampling point between the two consecutive extreme points. Determine whether the first geometric figure satisfies the preset first criterion. If it does not satisfy the criterion, the current transformer is considered to be saturated. The second criterion module is used to generate a second geometric figure and a third geometric figure respectively based on the three consecutive extreme points of the secondary side current and the midpoint of the sampling points between adjacent extreme points if the conditions are met. According to the preset second criterion, it is determined whether the second geometric figure and the third geometric figure are similar. If they are similar, the current transformer is considered to be unsaturated; otherwise, the current transformer is considered to be saturated.

[0011] Furthermore, the second criterion module is also used to take the three consecutive extreme points of the secondary side current as the third extreme point, the fourth extreme point and the fifth extreme point respectively, take the midpoint of the sampling points between the third extreme point and the fourth extreme point as the second midpoint, and take the midpoint of the sampling points between the fourth extreme point and the fifth extreme point as the third midpoint. Connect the third, fourth, and fifth extreme points one after the other in sequence to generate a second geometric figure; Connect the second midpoint, the fourth extreme point, and the third midpoint sequentially end to end to generate the third geometric figure.

[0012] Furthermore, the second criterion module is also used to determine the corresponding angles and corresponding sides between the second geometric figure and the third geometric figure based on the corresponding vertices between the second geometric figure and the third geometric figure; Determine whether the difference between the corresponding angles of the second geometric figure and the corresponding angles of the third geometric figure meets the angle difference threshold range, and whether the length ratio between the corresponding side of the second geometric figure and the corresponding side of the third geometric figure meets the side ratio threshold range. If the conditions are met, the second geometric figure and the third geometric figure are considered similar, and the current transformer is not saturated; otherwise, the second geometric figure and the third geometric figure are considered dissimilar, and the current transformer is saturated.

[0013] Thirdly, embodiments of the present invention also provide a computer device, including a memory, a processor, and a computer program stored in the memory and executable on the processor, wherein the processor executes the computer program to implement the steps of the above-described method.

[0014] Fourthly, embodiments of the present invention also provide a computer-readable storage medium having a computer program stored thereon, wherein the computer program, when executed by a processor, implements the steps of the above-described method.

[0015] This invention provides a graphical current transformer (CT) saturation identification method, system, device, and medium. Through distortion analysis based on local geometric features of the waveform, this invention enables continuous monitoring of CT saturation and desaturation processes. The algorithm is simple, computationally efficient, sensitive to waveform distortion, and independent of harmonic components and system parameter changes, exhibiting strong universality and engineering application value. This invention utilizes a first criterion for rapid CT saturation identification within the first half-wave after a fault, and a second criterion for accurate confirmation of CT saturation. This dual-criterion approach ensures both the speed of differential protection and the reliability of the criteria, achieving continuous and reliable monitoring of the entire CT saturation process after a fault, overcoming the limitations of a single criterion in response speed and reliability. Attached Figure Description

[0016] Figure 1 This is a flowchart illustrating the current transformer saturation identification method based on graphics in an embodiment of the present invention. Figure 2 This is a schematic diagram of the geometry of the first criterion when the CT outside the fault zone is not saturated in an embodiment of the present invention; Figure 3 This is a schematic diagram of the geometry of the first criterion for CT saturation in an external fault area according to an embodiment of the present invention; Figure 4 This is a schematic diagram of the geometry of the second criterion when the CT outside the fault zone is not saturated in an embodiment of the present invention; Figure 5 This is a schematic diagram of the geometry of the second criterion for CT saturation in an external fault scenario according to an embodiment of the present invention; Figure 6 This is a schematic diagram of the secondary side waveform when the CT is saturated due to an external fault in an embodiment of the present invention; Figure 7 This is a schematic diagram of the geometry of the first criterion under the slightly saturated state of the CT with an external fault in an embodiment of the present invention; Figure 8 This is a schematic diagram of the geometry of the second criterion under the slightly saturated state of the CT with an external fault in an embodiment of the present invention; Figure 9 This is a schematic diagram of the geometry of the first criterion under the transient saturation state of the CT with an external fault in an embodiment of the present invention; Figure 10 This is a schematic diagram of the geometry of the second criterion under the transient saturation state of the CT with an external fault in an embodiment of the present invention; Figure 11 This is a schematic diagram of the geometry of the first criterion under the concealed saturation state of the CT for external faults in an embodiment of the present invention; Figure 12 This is a schematic diagram of the geometry of the second criterion under the hidden saturation state of the CT outside the region in an embodiment of the present invention; Figure 13 This is a schematic diagram of the structure of the current transformer saturation identification system based on graphics in an embodiment of the present invention; Figure 14 This is an internal structural diagram of the computer device in an embodiment of the present invention.

[0017] Figure label: 10. First criterion module; 20. Second criterion module. Detailed Implementation

[0018] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, 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, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0019] Please see Figure 1The first embodiment of the present invention proposes a graph-based current transformer saturation identification method, including steps S10 to S40: Step S10: Sample the secondary current of the current transformer after the fault occurs, and generate a first geometric figure based on two consecutive extreme points of the secondary current and the midpoint of the sampling point between the two consecutive extreme points. Step S20: Determine whether the first geometric shape satisfies the preset first criterion. If it does not satisfy the criterion, the current transformer is considered to be saturated. Step S30: If satisfied, then generate the second geometric figure and the third geometric figure respectively based on the three consecutive extreme points of the secondary side current and the midpoint of the sampling points between adjacent extreme points. Step S40: According to the preset second criterion, determine whether the second geometric figure and the third geometric figure are similar. If they are similar, the current transformer is considered not saturated; otherwise, the current transformer is considered saturated.

[0020] This embodiment provides a method for identifying current transformer saturation to avoid protection maloperation caused by current transformer (CT) saturation due to external faults. The working principle of a current transformer is essentially that it transforms a high primary current into a low secondary current. Current transformer saturation is due to the nonlinear impedance characteristics of the current transformer core. When the primary current is too large, the core magnetic flux increases, causing a significant decrease in its permeability, which in turn reduces the equivalent impedance of its excitation branch, resulting in a significant increase in the excitation current. At this time, the secondary current is significantly distorted compared to the primary current. Therefore, the direct cause of current transformer saturation is the increase in excitation current caused by excessive core magnetic flux, which in turn is caused by excessive primary current (or current containing non-periodic components) or excessive secondary load.

[0021] Because CT saturation requires a flux accumulation process, there is a brief linear transmission region between the occurrence of a fault and the appearance of saturation. After this, the CT saturates, and the secondary current waveform exhibits obvious defects and begins to distort. During the saturation period, the degree of CT saturation fluctuates periodically with changes in the instantaneous value of the primary current, and the degree of waveform distortion also changes dynamically until the waveform gradually returns to normal after the CT exits saturation.

[0022] Based on the physical characteristics of the secondary current waveform change when the CT is saturated, it can be seen that waveform distortion will inevitably cause the position of the extreme point and the midpoint between adjacent extreme points to shift, thereby destroying the inherent geometric relationship of the waveform when the CT is not saturated. Therefore, this embodiment identifies the CT saturation state by detecting the geometric distortion of the secondary current waveform.

[0023] Speed ​​of operation refers to the ability of a relay protection device to quickly clear a short-circuit fault in a power system. To improve the speed of operation of differential protection, this embodiment introduces a first criterion based on adjacent extreme points and their midpoint. A first geometric figure is constructed by two consecutive extreme points and their midpoint, and the saturation of the CT is identified by determining whether the first geometric figure satisfies the first criterion. The steps for generating the first geometric figure include: The two consecutive extreme points of the secondary current and the midpoint of the sampling point between the two consecutive extreme points are respectively taken as the first extreme point, the second extreme point and the first midpoint; Connect the first extreme point, the first midpoint, and the second extreme point sequentially end to end to generate the first geometric shape.

[0024] In this embodiment, the secondary current of the current transformer is sampled after a fault occurs, and two adjacent extreme points are identified, namely the first extreme point and the second extreme point. The number of sampling points between the two adjacent extreme points is determined according to the sampling frequency. The median value of the number of sampling points is taken as the midpoint, namely the first midpoint. The first extreme point, the first midpoint, and the second extreme point are connected end to end to form the first geometric figure. It should be noted that coordinate transformation is introduced in the generation process of the geometric figure in this embodiment and subsequent embodiments. The time axis is multiplied by a scaling factor and normalized so that the geometric length of 1 ms is equivalent to the geometric length of 1A. By unifying the dimensions of time and current, the subsequent calculation of side length and angle has a clear physical meaning.

[0025] After generating the first geometric figure, determine whether the first geometric figure satisfies the first criterion. The specific steps include: The line connecting the first extreme point and the first midpoint is taken as the first line, the line connecting the second extreme point and the first midpoint is taken as the second line, and the line connecting the first extreme point and the second extreme point is taken as the third line. The ratio of the length of the first connecting line to the length of the second connecting line is used as the first indicator, and the ratio of the first indicator to the length of the third connecting line is used as the second indicator. Determine whether the first indicator meets the threshold range of the first indicator and whether the second indicator meets the threshold range of the second indicator. If not, the current transformer is considered to be saturated.

[0026] In this embodiment, the first criterion is set to quickly identify waveform distortion by judging whether the three points of the first geometric figure form a geometric triangle. When the external fault CT is not saturated, the geometric figure of the first criterion formed by the extreme points and midpoints of the secondary current waveform is as follows: Figure 2As shown, it can be seen that adjacent extreme points A and B in the secondary current waveform are strictly collinear with their midpoint M1, and do not form a triangle. Furthermore, M1 is the midpoint of the line AB connecting extreme points A and B. When the external fault CT is saturated, the geometric figure of the first criterion formed by the extreme points and midpoint of the secondary current waveform is as follows: Figure 3 As shown, due to CT saturation causing waveform defects, the midpoint M1 will shift. At this time, the extreme point A, the midpoint M1, and the extreme point B form a triangle, and AM1 ≠ BM1. Therefore, when △AM1B exists, that is, when the first geometric figure is a triangle, it means that CT is saturated. Based on this principle, the first criterion constructed in this embodiment can be expressed as: In the formula, |AM1| represents the length of the line AM1 connecting the first extreme point A and the first midpoint M1, |BM1| represents the length of the line BM1 connecting the second extreme point B and the first midpoint M1, and |AB| represents the length of the line AB connecting the first extreme point A and the second extreme point B. a, b, and c are all preset thresholds. Preferably, a is 0.9, and b and c are both 1.1.

[0027] The first expression of the first criterion is used to determine whether the first midpoint M1 is the midpoint of the line AB. The second expression is used to determine whether the extreme point and the midpoint are collinear. If both expressions are satisfied, the extreme point and the midpoint are considered to be collinear. If either expression is not satisfied, the first geometric figure is considered to be a triangle, which conforms to the waveform characteristics when the CT is saturated. Therefore, the CT can be determined to be saturated, and the differential protection is immediately locked.

[0028] This embodiment only requires two consecutive extreme points and their midpoint. By judging whether the three points form a geometric triangle, waveform distortion can be quickly identified, so that saturation discrimination can be initiated within the first half-wave after the fault, which can effectively improve the speed of differential protection.

[0029] Although a triangular first geometric figure signifies CT saturation, the first geometric figure does not necessarily always exist when the CT is saturated; that is, a triangular first geometric figure and CT are sufficient but not necessary conditions. To further accurately determine whether the CT is saturated, this embodiment provides a second criterion based on the first criterion. In the second criterion, the second and third geometric figures are generated firstly using three consecutive extreme points of the secondary current after the fault occurs and the midpoint of the sampled values ​​between adjacent extreme points. Based on the second criterion, the similarity between the second and third geometric figures is analyzed. The steps for generating the second and third geometric figures include: The three consecutive extreme points of the secondary current are respectively designated as the third extreme point, the fourth extreme point, and the fifth extreme point. The midpoint of the sampling points between the third extreme point and the fourth extreme point is designated as the second midpoint, and the midpoint of the sampling points between the fourth extreme point and the fifth extreme point is designated as the third midpoint. Connect the third, fourth, and fifth extreme points one after the other in sequence to generate a second geometric figure; Connect the second midpoint, the fourth extreme point, and the third midpoint sequentially end to end to generate the third geometric figure.

[0030] In this embodiment, three consecutive extreme points of the sampled secondary current are connected end-to-end to form a second geometric figure. Simultaneously, the two midpoints of the sampling points between adjacent extreme points, as well as the extreme points between those midpoints, are connected end-to-end to form a third geometric figure. This can be understood as generating a larger triangle and a smaller triangle based on the three consecutive extreme points of the secondary current and the midpoints of the sampling points between adjacent extreme points. Then, the similarity between these two triangles is determined according to a preset second criterion. Specific steps include: Based on the corresponding vertices between the second geometric figure and the third geometric figure, determine the corresponding angles and corresponding sides between the second geometric figure and the third geometric figure; Determine whether the difference between the corresponding angles of the second geometric figure and the corresponding angles of the third geometric figure meets the angle difference threshold range, and whether the length ratio between the corresponding side of the second geometric figure and the corresponding side of the third geometric figure meets the side ratio threshold range. If the conditions are met, the second geometric figure and the third geometric figure are considered similar, and the current transformer is not saturated; otherwise, the second geometric figure and the third geometric figure are considered dissimilar, and the current transformer is saturated.

[0031] In this embodiment, the second criterion uses two triangles formed by three consecutive extreme points and the midpoints of the sampling points between adjacent extreme points to analyze the geometric distortion of the secondary current waveform. Assuming that the second geometric figure formed by the three consecutive extreme points C, D and E is △CDE, and the third geometric figure formed by the midpoint M2 of the sampling points between extreme point C and extreme point D, the midpoint M3 of the sampling points between extreme point D and extreme point E is △M2DM3, that is, the corresponding vertex of the two triangles is extreme point D, the corresponding angles are ∠CDE and ∠M2DM3, and the corresponding sides are CD and M2D, and DE and DM3, respectively.

[0032] When the external fault CT is not saturated, the geometric figure of the second criterion formed by the extreme points and midpoints of the secondary current waveform is as follows: Figure 4As shown, △CDE and △M2DM3 satisfy a geometric similarity relationship, and the ratio of their corresponding sides is always 2, i.e., |CD| / |M2D|=2, and |CD| / |DM3|=2. Here, |M2D| represents the length of the line M2D connecting the extreme point D and the midpoint M2, |DM3| represents the length of the line DM3 connecting the extreme point D and the midpoint M3, and |CD| represents the length of the line CD connecting the extreme point C and the extreme point D. When the external fault CT is saturated, the geometric figure of the second criterion formed by the extreme points and midpoints of the secondary current waveform is as follows: Figure 5 As shown, when CT saturates, the midpoint shifts, and the similarity between the two geometric figures is destroyed. Therefore, this embodiment uses a second criterion to determine whether the two geometric figures satisfy a similarity relationship, the expression of which is as follows: In the formula, β1, β2, γ1, γ2 and δ are all preset thresholds. Preferably, β1 and γ1 are both 1.9, β2 and γ2 are both 2.1 and δ is 3°.

[0033] In the second criterion mentioned above, the first two expressions are used to determine whether the ratios between corresponding sides CD and M2D, and DE and DM3 of the two triangles have changed, while the third expression is used to determine whether the angles of corresponding angles of the two triangles have changed.

[0034] If the second and third geometric figures satisfy the second criterion, it indicates that the two triangles are similar, and therefore CT is considered unsaturated. Conversely, if any expression is not satisfied, it indicates that the midpoint has shifted, and the similarity of the triangles is broken; in this case, CT is considered saturated. The second criterion in this embodiment has a clear physical meaning and can capture the change in the geometric relationship of the waveform before and after saturation, thereby achieving simple and efficient CT saturation identification. It should be noted that even if CT saturation is determined by the first criterion, the second criterion can still be used to further confirm saturation. In addition, depending on the actual situation of current acquisition, the similarity of the second criterion can be determined first, followed by the determination of the first criterion. If any criterion is not satisfied, CT saturation can be determined.

[0035] The following example illustrates the specific determination process of this embodiment using a fault scenario where an external fault is accompanied by hidden CT saturation. When the CT saturates and, with the accumulation of magnetic flux, the CT operates almost entirely above the core saturation inflection point, the permeability and equivalent excitation reactance are basically constant, with no alternation between saturation and desaturation. The second harmonic content of the secondary current of the CT is very small. At this time, the electrical characteristics are completely different from the electrical characteristics of conventional transient saturation. This embodiment defines this saturation state as hidden CT saturation. Hidden saturation is a saturation state under special operating conditions and is not necessarily present. However, under this saturation state, the conventional second harmonic method cannot accurately determine the saturation state of the CT.

[0036] Taking a three-phase short-circuit fault outside the fault zone occurring on the corner side of transformer Y / D-11 in a 110kV substation at t=0.3935s as an example, the low-voltage side C-phase CT enters a slightly saturated state 22.5ms after the fault, and its secondary side waveform is as follows: Figure 6 As shown, the first criterion completes the discrimination at t=0.42s, and its geometric figure is as follows. Figure 7 As shown, the first geometric shape is determined to be a triangle, at which point the CT is saturated and the differential protection is locked. Subsequently, when three consecutive extreme points are obtained, the second criterion is used for discrimination, and its geometric shape is as follows. Figure 8 As shown, the second geometric figure is determined to be dissimilar to the third geometric figure, further confirming CT saturation.

[0037] Subsequently, the sliding window continuously monitors the fault current. Due to the deepening of magnetic flux saturation, the CT enters the normal transient saturation stage. During the transient saturation stage, the first criterion is executed, and its geometry is as follows. Figure 9 As shown, the first geometric shape is determined to be a triangle. At this point, the CT is saturated, and the protection is locked. Subsequently, the second criterion is applied, and its geometric shape is as follows. Figure 10 As shown, the second geometric figure is determined to be dissimilar to the third geometric figure, confirming CT saturation.

[0038] As the magnetic flux continues to accumulate, the CT operates entirely above the core saturation point, entering a hidden saturation state. At this point, the second harmonic content of the secondary current is very small, and the harmonic blocking criterion cannot reliably operate. However, in this scheme, the first and second criteria can still be used for discrimination. At t=0.4502s, the first criterion completes the discrimination, and its geometric figure is as follows. Figure 11 As shown, the first geometric shape is determined to be a triangle, indicating CT saturation and protection locking; subsequently, the second criterion is applied, and its geometric shape is as follows. Figure 12 As shown, the second and third geometric figures are determined to be dissimilar, confirming CT saturation. It can be clearly seen that this solution can still accurately and quickly identify out-of-area faults even when CT saturation occurs and the second harmonic criterion fails, effectively compensating for the shortcomings of traditional second harmonic criterion.

[0039] It should be noted that in the actual judgment process of this embodiment, the two criteria can be understood as parallel judgments. However, since the first criterion only requires half-wave data, while the second criterion requires full-wave data, the first criterion will inevitably be activated first after a fault occurs. At this time, the second criterion sets the output result to similarity according to the initial value of the program. If the second half-wave has already saturated after the fault, the waveform is distorted, and the first criterion completes the judgment and outputs the existence of a triangle, at which point CT saturation lockout is met, satisfying the speed requirement. If the data in the second half-wave of the fault is insufficient to activate the triangle similarity criterion, the similarity criterion only outputs the default value "similar". This is mainly because the traditional time-difference method has a very short initial linear transmission zone and a small time difference when the CT is rapidly saturated, making it easy to miss detections. However, the first criterion is very sensitive to rapid saturation and can reliably identify the situation where rapid saturation occurs within the second half-cycle of the fault.

[0040] If the first criterion fails to identify CT saturation due to slight distortion in the initial half-wave waveform, after obtaining the cycle data, both criteria will be used together for a single determination (initial half-wave obtains extreme points 1 and 2, then the cycle data adds another extreme point 3, making it three extreme points 1, 2, and 3; the existence criterion for extreme point 1 and 2 indicates the triangle does not exist; at the third extreme point, extreme points 2 and 3 will be determined by the existence criterion, while extreme points 1, 2, and 3 will be determined by the similarity criterion). Based on the results of the two criteria, if either criterion is not met, saturation is determined, and the differential protection is blocked. At this point, the two criteria operate in parallel using sliding time windows.

[0041] The first criterion is primarily for situations where saturation occurs in the latter half of a fault cycle, as insufficient data makes similarity judgments impossible at this point. Therefore, the first criterion is used to improve the speed of response. If saturation occurs one cycle after the fault, both criteria are used together for each extreme point to ensure no missed detections. This execution logic enables reliable and rapid response for various operating conditions.

[0042] This embodiment provides a graphical current transformer (CT) saturation identification method. Through distortion analysis based on local geometric features of the waveform, it can continuously monitor the CT saturation and desaturation process. The algorithm is simple, computationally inexpensive, sensitive to waveform distortion, and independent of harmonic components. It is unaffected by changes in system parameters and has strong universality and engineering application value. This embodiment uses a first criterion to quickly identify CT saturation within the first half-wave after a fault and a second criterion to accurately confirm CT saturation. The dual criteria ensure both the speed of differential protection and the reliability of the criteria, achieving continuous and reliable monitoring of the entire CT saturation process after a fault occurs, overcoming the limitations of a single criterion in response speed and reliability.

[0043] Please see Figure 13Based on the same inventive concept, the second embodiment of the present invention proposes a graph-based current transformer saturation identification system, comprising: The first criterion module 10 is used to sample the secondary current of the current transformer after the fault occurs, and generate a first geometric figure based on two consecutive extreme points of the secondary current and the midpoint of the sampling point between the two consecutive extreme points. Determine whether the first geometric figure satisfies the preset first criterion. If it does not satisfy the criterion, the current transformer is considered to be saturated. The second criterion module 20 is used to generate a second geometric figure and a third geometric figure respectively based on the three consecutive extreme points of the secondary side current and the midpoint of the sampling points between adjacent extreme points if the conditions are met. According to the preset second criterion, it is determined whether the second geometric figure and the third geometric figure are similar. If they are similar, the current transformer is considered to be unsaturated; otherwise, the current transformer is considered to be saturated.

[0044] The technical features and effects of the graph-based current transformer saturation identification system proposed in this invention are the same as those of the method proposed in this invention, and will not be repeated here. Each module in the above-mentioned graph-based current transformer saturation identification system can be implemented entirely or partially through software, hardware, or a combination thereof. These modules can be embedded in or independent of the processor in a computer device in hardware form, or stored in the memory of a computer device in software form, so that the processor can call and execute the operations corresponding to each module.

[0045] Furthermore, embodiments of the present invention also propose a computer device, including a memory, a processor, and a computer program stored in the memory and executable on the processor, wherein the processor executes the computer program to implement the steps of the above-described method.

[0046] Please see Figure 14The diagram illustrates the internal structure of a computer device in one embodiment. This computer device can specifically be a terminal or a server. The computer device includes a processor, memory, network interface, display, and input devices connected via a system bus. The processor provides computing and control capabilities. The memory includes a non-volatile storage medium and internal memory. The non-volatile storage medium stores an operating system and computer programs. The internal memory provides an environment for the operation of the operating system and computer programs in the non-volatile storage medium. The network interface of the computer device is used for communication with external terminals via a network connection. When the computer program is executed by the processor, it implements a graphics-based current transformer saturation identification method. The display screen of the computer device can be a liquid crystal display (LCD) or an e-ink display. The input devices can be a touch layer covering the display screen, buttons, a trackball, or a touchpad mounted on the computer device's casing, or an external keyboard, touchpad, or mouse.

[0047] Those skilled in the art will understand that Figure 14 The structure shown is merely a block diagram of a portion of the structure related to the present application and does not constitute a limitation on the computer device to which the present application is applied. Specific computing devices may include more or fewer components than those shown in the figure, or combine certain components, or have the same component arrangement.

[0048] Furthermore, embodiments of the present invention also propose a computer-readable storage medium storing a computer program thereon, which, when executed by a processor, implements the steps of the above-described method.

[0049] In summary, the embodiments of this invention propose a graphic-based method, system, device, and medium for identifying current transformer saturation. The method samples the secondary current of the current transformer after a fault occurs. Based on two consecutive extreme points of the secondary current and the midpoint of the sampling point between the two consecutive extreme points, a first geometric figure is generated. It then determines whether the first geometric figure satisfies a preset first criterion. If it does not, the current transformer is considered saturated. If it does, a second and third geometric figure are generated based on three consecutive extreme points of the secondary current and the midpoint of the sampling point between adjacent extreme points. Finally, based on a preset second criterion, it is determined whether the second and third geometric figures are similar. If they are similar, the current transformer is considered not saturated; otherwise, it is considered saturated. This invention enables continuous monitoring of CT saturation and desaturation processes through distortion analysis based on local geometric features of the waveform. The algorithm is simple, computationally efficient, sensitive to waveform distortion, and independent of harmonic components and system parameter changes, exhibiting strong universality and engineering application value. This invention utilizes a first criterion to rapidly identify CT saturation within the first half-wave after a fault, and a second criterion to accurately confirm CT saturation. This dual criterion ensures both the speed of differential protection and the reliability of the criteria, achieving continuous and reliable monitoring of the entire CT saturation process after a fault occurs, overcoming the limitations of a single criterion in response speed and reliability.

[0050] The various embodiments in this specification are described in a progressive manner. For directly identical or similar parts of the embodiments, refer to each other. Each embodiment focuses on its differences from other embodiments. In particular, the system embodiments are basically similar to the method embodiments, so the description is relatively simple; relevant parts can be referred to the descriptions in the method embodiments. It should be noted that the technical features of the above embodiments can be combined arbitrarily. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as the combination of these technical features does not contradict each other, it should be considered within the scope of this specification.

[0051] The embodiments described above are merely preferred embodiments of this application, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various improvements and substitutions without departing from the technical principles of this invention, and these improvements and substitutions should also be considered within the scope of protection of this application. Therefore, the scope of protection of this patent application should be determined by the scope of the claims.

Claims

1. A graph-based method for identifying saturation in current transformers, characterized in that, include: The secondary current of the current transformer after the fault occurs is sampled, and a first geometric figure is generated based on two consecutive extreme points of the secondary current and the midpoint of the sampling point between the two consecutive extreme points. Determine whether the first geometric figure satisfies the preset first criterion. If it does not satisfy the criterion, the current transformer is considered to be saturated. If satisfied, then based on the three consecutive extreme points of the secondary side current and the midpoint of the sampling points between adjacent extreme points, the second and third geometric figures are generated respectively. According to the preset second criterion, it is determined whether the second geometric figure and the third geometric figure are similar. If they are similar, the current transformer is considered to be unsaturated; otherwise, the current transformer is considered to be saturated.

2. The graph-based current transformer saturation identification method according to claim 1, characterized in that, The step of generating the first geometric figure based on two consecutive extreme points of the secondary current and the midpoint of the sampling point between the two consecutive extreme points includes: The two consecutive extreme points of the secondary current and the midpoint of the sampling point between the two consecutive extreme points are respectively taken as the first extreme point, the second extreme point and the first midpoint; Connect the first extreme point, the first midpoint, and the second extreme point sequentially end to end to generate the first geometric shape.

3. The graph-based current transformer saturation identification method according to claim 2, characterized in that, The step of determining whether the first geometric shape satisfies a preset first criterion, and considering the current transformer to be saturated if it does not, includes: The line connecting the first extreme point and the first midpoint is taken as the first line, the line connecting the second extreme point and the first midpoint is taken as the second line, and the line connecting the first extreme point and the second extreme point is taken as the third line. The ratio of the length of the first connecting line to the length of the second connecting line is used as the first indicator, and the ratio of the first indicator to the length of the third connecting line is used as the second indicator. Determine whether the first indicator meets the threshold range of the first indicator and whether the second indicator meets the threshold range of the second indicator. If not, the current transformer is considered to be saturated.

4. The graph-based current transformer saturation identification method according to claim 1, characterized in that, The step of generating the second and third geometric figures based on the three consecutive extreme points of the secondary current and the midpoint of the sampling points between adjacent extreme points includes: The three consecutive extreme points of the secondary current are respectively designated as the third extreme point, the fourth extreme point, and the fifth extreme point. The midpoint of the sampling points between the third extreme point and the fourth extreme point is designated as the second midpoint, and the midpoint of the sampling points between the fourth extreme point and the fifth extreme point is designated as the third midpoint. Connect the third, fourth, and fifth extreme points one after the other in sequence to generate a second geometric figure; Connect the second midpoint, the fourth extreme point, and the third midpoint sequentially end to end to generate the third geometric figure.

5. The graph-based current transformer saturation identification method according to claim 4, characterized in that, The step of determining whether the second geometric figure and the third geometric figure are similar according to a preset second criterion, and if they are similar, then the current transformer is considered not saturated; otherwise, the current transformer is considered saturated. Based on the corresponding vertices between the second geometric figure and the third geometric figure, determine the corresponding angles and corresponding sides between the second geometric figure and the third geometric figure; Determine whether the difference between the corresponding angles of the second geometric figure and the corresponding angles of the third geometric figure meets the angle difference threshold range, and whether the length ratio between the corresponding side of the second geometric figure and the corresponding side of the third geometric figure meets the side ratio threshold range. If the conditions are met, the second geometric figure and the third geometric figure are considered similar, and the current transformer is not saturated; otherwise, the second geometric figure and the third geometric figure are considered dissimilar, and the current transformer is saturated.

6. A graph-based current transformer saturation identification system, characterized in that, include: The first criterion module is used to sample the secondary current of the current transformer after the fault occurs, and generate a first geometric figure based on two consecutive extreme points of the secondary current and the midpoint of the sampling point between the two consecutive extreme points. Determine whether the first geometric figure satisfies the preset first criterion. If it does not satisfy the criterion, the current transformer is considered to be saturated. The second criterion module is used to generate a second geometric figure and a third geometric figure respectively based on the three consecutive extreme points of the secondary side current and the midpoint of the sampling points between adjacent extreme points if the conditions are met. According to the preset second criterion, it is determined whether the second geometric figure and the third geometric figure are similar. If they are similar, the current transformer is considered to be unsaturated; otherwise, the current transformer is considered to be saturated.

7. The graph-based current transformer saturation identification system according to claim 6, characterized in that, The second criterion module is further configured to take the three consecutive extreme points of the secondary side current as the third extreme point, the fourth extreme point and the fifth extreme point respectively, take the midpoint of the sampling points between the third extreme point and the fourth extreme point as the second midpoint, and take the midpoint of the sampling points between the fourth extreme point and the fifth extreme point as the third midpoint. Connect the third, fourth, and fifth extreme points one after the other in sequence to generate a second geometric figure; Connect the second midpoint, the fourth extreme point, and the third midpoint sequentially end to end to generate the third geometric figure.

8. The graph-based current transformer saturation identification system according to claim 7, characterized in that, The second criterion module is further configured to determine the corresponding angles and corresponding sides between the second geometric figure and the third geometric figure based on the corresponding vertices between the second geometric figure and the third geometric figure; Determine whether the difference between the corresponding angles of the second geometric figure and the corresponding angles of the third geometric figure meets the angle difference threshold range, and whether the length ratio between the corresponding side of the second geometric figure and the corresponding side of the third geometric figure meets the side ratio threshold range. If the conditions are met, then the second geometric figure and the third geometric figure are considered similar, and the current transformer is not saturated; Conversely, if the second and third geometric figures are not similar, the current transformer is considered to be saturated.

9. A computer device, comprising a memory, a processor, and a computer program stored in the memory and executable on the processor, characterized in that, When the processor executes the computer program, it implements the steps of the method according to any one of claims 1 to 5.

10. A computer-readable storage medium having a computer program stored thereon, characterized in that, When the computer program is executed by a processor, it implements the steps of the method according to any one of claims 1 to 5.

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