A method for protecting an electrical grid based on an improved teager energy operator

Abstract

This invention discloses a boundaryless power grid protection method based on an improved Teager energy operator, comprising: initiating protection based on the line-mode voltage gradient; calculating the forward traveling wave transient energy and the reverse traveling wave transient energy based on the line-mode voltage, line-mode current, and line-mode wave impedance collected before and after the initiation time, and determining whether it is a forward fault; when a forward fault is determined, normalizing the line-mode voltage and removing the first two sampling points within a fixed time window, calculating the Teager energy value for each remaining sampling point after removal, taking the maximum value among all Teager energy values ​​as the maximum value of the line-mode voltage energy, and determining whether the fault is inside or outside the fault zone; when a fault is determined to be inside the fault zone, performing protection action to trip; when a fault is determined to be outside the fault zone, blocking the protection. This invention can effectively solve the problem of difficulty in fault zone identification caused by the weakening of boundary effects in boundaryless multi-terminal flexible DC transmission systems.

Description

Technical Field

[0001] This invention belongs to the field of high voltage direct current transmission technology in power systems, and particularly relates to a boundaryless grid protection method based on an improved Teager energy operator. Background Technology

[0002] Multi-terminal flexible DC power grids, with their flexible and efficient power transmission capabilities, are considered an important component of future power grid development. However, these grids suffer from prominent problems such as extremely rapid fault current rise rates and severe transient processes. To effectively limit fault currents, current engineering practices commonly employ the method of installing current-limiting reactors at both ends of the line. These reactors not only physically constitute "boundary" elements between adjacent lines but also provide a key technological breakthrough for achieving selective line protection.

[0003] As the topology of flexible DC power grids becomes increasingly complex, a "borderless grid" structure, which centrally configures current-limiting reactors at the converter station outlet, is showing great development potential due to its significant economic and structural advantages. It is foreseeable that future power grid structures will evolve towards a "borderless grid."

[0004] In response to the differences in electrical quantities exhibited by boundary elements such as current-limiting reactors when faults occur inside and outside the fault zone, scholars both domestically and internationally have proposed various protection schemes. Generally speaking, existing mature protection strategies primarily utilize the differences in voltage, current, and line parameters on both sides of the boundary element to identify the fault zone, demonstrating good selectivity in practice.

[0005] However, the aforementioned protection strategies based on boundary elements are highly dependent on the existence of the "boundary" itself. In boundaryless power grids, since the boundary effect is significantly reduced or even nonexistent, it is still uncertain whether the original protection methods based on boundary characteristics can be directly applied, and they urgently need to be re-examined and redesigned. Specifically, when applied to boundaryless multi-terminal DC transmission systems, existing technologies mainly reveal the following shortcomings: Traditional boundary protection methods fail: In a boundaryless power grid, there is a lack of current-limiting reactors that form obvious points of impedance discontinuity, making it difficult for traditional protection principles based on the differences of boundary elements to effectively distinguish between faults inside and outside the zone.

[0006] Insufficient tolerance to transition resistance and noise: When encountering high-resistance faults, some existing protection schemes exhibit weak fault characteristics, which can easily lead to protection failure. At the same time, their resistance to electromagnetic interference is weak, and their reliability decreases in noisy environments.

[0007] Over-reliance on reliable communication: Some protection schemes that rely on communication coordination or two-way communication not only increase system complexity and cost, but their performance is also limited by the reliability and latency of the communication channel.

[0008] Therefore, in response to the prominent problems such as the weakening of boundary effects in borderless power grids and the insufficient anti-interference capability of existing protection methods, it is urgent to propose a traveling wave protection method for DC lines in borderless power grids based on an improved Teager energy operator. Summary of the Invention

[0009] To address the aforementioned technical problems, this invention provides a boundaryless power grid protection method based on an improved Teager energy operator, comprising the following steps: Obtain the line-mode voltage and line-mode current at the protection measuring point; The protection is activated based on whether the line-mode voltage gradient exceeds the activation threshold value. In response to protection activation, based on the line-mode voltage, line-mode current, and line-mode impedance collected before and after activation, the forward traveling wave transient energy and the reverse traveling wave transient energy are calculated, and it is determined whether it is a positive direction fault based on whether the ratio of the forward traveling wave transient energy to the reverse traveling wave transient energy is greater than the direction identification threshold. When a positive fault is determined, the line mode voltage is normalized, and the first two sampling points used for fault direction determination are removed within a fixed time window. The Teager energy value is calculated for each remaining sampling point after removal, and the maximum value among all Teager energy values ​​is taken as the maximum energy value of the line mode voltage. Based on the comparison between the maximum value of the line mode voltage energy and the energy threshold value, it is determined whether it is an in-zone fault or an out-of-zone fault; When a fault is determined to be within the zone, the protection system trips; when a fault is determined to be outside the zone, the protection system is locked.

[0010] Optionally, the process of initiating protection based on whether the line-mode voltage gradient exceeds the activation threshold includes: Calculate the absolute value of the difference between the line-mode voltage at the current sampling time and the line-mode voltage at the previous sampling time, and use the obtained absolute value of the difference as the line-mode voltage gradient; when the line-mode voltage gradient is greater than or equal to the preset voltage gradient start threshold value, the protection is determined to start.

[0011] Optionally, the process of calculating the transient energy of the forward traveling wave and the transient energy of the reverse traveling wave includes: The instantaneous values ​​of the forward traveling wave and the reverse traveling wave are calculated based on the fault components of the line-mode voltage and the line-mode current, as well as the line-mode wave impedance. Within two sampling points after the start-up time, the sum of the squares of the instantaneous values ​​of the forward traveling wave is calculated as the transient energy of the forward traveling wave, and the sum of the squares of the instantaneous values ​​of the reverse traveling wave is calculated as the transient energy of the reverse traveling wave. The ratio of the transient energy of the forward traveling wave to the transient energy of the reverse traveling wave is calculated. When this ratio is greater than the direction recognition threshold, it is determined to be a positive direction fault.

[0012] Optionally, the process of normalizing the line-mode voltage includes: Determine the maximum and minimum values ​​in the line-mode voltage data; calculate the difference between the maximum and minimum values ​​as the range; when the range is greater than a preset range threshold, linearly map the data to a first interval with a first positive number as the lower limit and a second positive number as the upper limit; when the range is less than or equal to the preset range threshold, linearly map the data to a second interval with the first positive number as the lower limit and a third positive number as the upper limit, where the second positive number is greater than the third positive number.

[0013] Optionally, the process of determining whether a fault is within or outside the zone based on the comparison result between the maximum value of the line-mode voltage energy and the energy threshold value includes: The maximum line-mode voltage energy is compared with the energy threshold value; when the maximum line-mode voltage energy is greater than the energy threshold value, it is determined to be an external fault; when the maximum line-mode voltage energy is less than the energy threshold value, it is determined to be an internal fault.

[0014] Optionally, after determining that the fault is within the area, the method also includes fault polarity selection: Once a fault is identified as being within the zone, the sampled value of the zero-mode voltage within a fixed time window is obtained; the first two sampling points used for fault direction determination are deleted; the average value of the zero-mode voltage of the remaining sampling points is calculated; when the average value of the zero-mode voltage is less than the negative pole selection threshold, it is determined to be a positive pole fault; when the average value of the zero-mode voltage is greater than the positive pole selection threshold, it is determined to be a negative pole fault; when the average value of the zero-mode voltage is between the negative pole selection threshold and the positive pole selection threshold, it is determined to be a bipolar fault.

[0015] Optionally, it also includes a lightning interference detection step, the process of which includes: After protection is activated but before direction determination, the absolute values ​​of line-mode voltage fault components within a fixed time window are extracted. The fixed time window is divided into a first half and a second half. The sum of the absolute values ​​of line-mode voltage fault components at each sampling point within the first half and the sum of the absolute values ​​of line-mode voltage fault components at each sampling point within the second half are calculated. The ratio of the sum of the first half and the sum of the second half is calculated. When this ratio is greater than the product of the preset lightning strike criterion reliability coefficient and the ratio of the fault voltage amplitude at the end of the line, it is determined to be lightning interference and the protection is blocked.

[0016] Optionally, it also includes a protection dead zone elimination step, the process of which includes: When the local protection determines that the fault is outside the positive direction zone, the local protection is locked and does not operate; when the local protection receives a trip signal sent by the opposite protection, it immediately performs the trip operation; the condition for the opposite protection to send a trip signal is that the opposite protection determines that the fault is inside the zone and performs the trip operation.

[0017] On the other hand, the present invention also provides an electronic device including a memory, a processor, and a computing program stored in the memory and executable on the processor, wherein the processor implements the method when executing the computing program.

[0018] On the other hand, the present invention also provides a computer-readable storage medium storing a computer program that, when executed by a processor, implements the method.

[0019] Compared with the prior art, the present invention has the following advantages and technical effects: This invention is based on the line-mode voltage and line-mode current collected at the protection measurement point. It achieves rapid start-up after a fault through voltage gradient and accurately determines the fault direction based on the transient energy ratio of the traveling wave and the reverse traveling wave. Thus, without relying on communication with the other side, the fault direction can be reliably identified using single-ended electrical quantities, which significantly improves the protection's operating speed and independence.

[0020] This invention addresses the characteristics of boundary elements being missing and traveling wave transmission effects being significant in boundaryless power grids. By adaptively normalizing the fault line mode voltage, the influence of amplitude differences under different fault conditions on protection criteria is eliminated. Combined with the method of calculating the maximum value of Teager energy after removing direction discrimination sampling points within a fixed time window, the high-frequency energy characteristics of traveling waves reflecting the differences between faults inside and outside the zone are effectively extracted, achieving accurate differentiation between faults inside and outside the zone and overcoming the defect of traditional boundary protection failing in boundaryless power grids.

[0021] This invention employs an improved Teager energy operator to construct the identification criterion between the inside and outside of the region. The calculation can be completed with only voltage. Furthermore, through normalization processing and fixed time window design, the criterion has a high tolerance to transition resistance and noise interference, ensuring the reliability of the protection under complex actual working conditions.

[0022] This invention directly trips the circuit breaker when the fault is determined to be within the zone and reliably blocks the circuit breaker when the fault is outside the zone, forming a complete single-ended main protection logic. It does not require waiting for information from the other side or the arrival of a second traveling wave, thus meeting the stringent requirements of flexible DC power grids for protection speed. At the same time, it provides an economical and effective engineering protection scheme for boundaryless multi-terminal DC transmission systems. Attached Figure Description

[0023] The accompanying drawings, which form part of this application, are used to provide a further understanding of this application. The illustrative embodiments and descriptions of this application are used to explain this application and do not constitute an undue limitation of this application. In the drawings: Figure 1This is a diagram of a ring-shaped power grid topology according to an embodiment of the present invention; wherein, (a) is a schematic diagram of a bounded power grid topology, and (b) is a schematic diagram of a boundless power grid topology; Figure 2 The following are inverse traveling wave waveform diagrams according to embodiments of the present invention; wherein, (a) is an inverse traveling wave waveform diagram of a grid fault with boundaries, and (b) is an inverse traveling wave waveform diagram of a grid fault without boundaries; Figure 3 This is the Peterson equivalent circuit diagram of an embodiment of the present invention; Figure 4 This is a diagram showing the amplitude-frequency response of the refractive index according to an embodiment of the present invention; Figure 5 This is a time-frequency characteristic diagram of the fault traveling wave according to an embodiment of the present invention; Figure 6 This is a diagram of the fault traveling wave propagation path according to an embodiment of the present invention; Figure 7 This is a diagram showing the difference in arrival time of the fault traveling wave according to an embodiment of the present invention. Figure 8 The following are fault traveling wave difference diagrams within the time window of this invention embodiment; wherein, (a) is a fault traveling wave difference diagram within the time window of an intra-region fault, and (b) is a fault traveling wave difference diagram within the time window of an extra-region fault. Figure 9 This is a diagram showing the line protection range according to an embodiment of the present invention; Figure 10 This is a flowchart of the protection scheme according to an embodiment of the present invention; Figure 11 This is a schematic diagram of a simulation model of a borderless ±500kV four-terminal flexible DC power grid according to an embodiment of the present invention. Detailed Implementation

[0024] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. This application will now be described in detail with reference to the accompanying drawings and embodiments.

[0025] It should be noted that the steps shown in the flowchart in the accompanying drawings can be executed in a computer system such as a set of computer-executable instructions, and although a logical order is shown in the flowchart, in some cases the steps shown or described may be executed in a different order than that shown here.

[0026] Example 1 This embodiment provides a boundaryless power grid protection method based on an improved Teager energy operator, including the following steps: Obtain the line-mode voltage and line-mode current at the protection measuring point; The protection is activated based on whether the line-mode voltage gradient exceeds the activation threshold value. In response to protection activation, based on the line-mode voltage, line-mode current, and line-mode impedance collected before and after activation, the forward traveling wave transient energy and the reverse traveling wave transient energy are calculated, and it is determined whether it is a positive direction fault based on whether the ratio of the forward traveling wave transient energy to the reverse traveling wave transient energy is greater than the direction identification threshold. When a positive fault is determined, the line mode voltage is normalized, and the first two sampling points used for fault direction determination are removed within a fixed time window. The Teager energy value is calculated for each remaining sampling point after removal, and the maximum value among all Teager energy values ​​is taken as the maximum energy value of the line mode voltage. Based on the comparison between the maximum value of the line mode voltage energy and the energy threshold value, it is determined whether it is an in-zone fault or an out-of-zone fault; When a fault is determined to be within the zone, the protection system trips; when a fault is determined to be outside the zone, the protection system is locked. As a specific implementation method, the following steps are included: Feasible, boundaryless power grid fault traveling wave propagation processes include: This embodiment takes the Zhangbei four-terminal flexible DC transmission system as an example, and its bounded and unbounded power grid topologies are as follows: Figure 1 As shown in (a) and (b) in the figure.

[0027] Among them are current-limiting reactors in the boundary power grid. The current-limiting reactors of the grid-free power grid are installed at both ends of the transmission line, and there are boundary elements between adjacent lines; while the current-limiting reactors of the grid-free power grid are installed at the outlet of the converter station, and there are no boundary elements between adjacent lines.

[0028] When a fault occurs in line 1, the protection and Record the reverse traveling wave waveform as follows Figure 2 As shown. In a boundary power grid, due to the presence of boundary elements, fault traveling waves are difficult to propagate to adjacent lines. When the fault reverse traveling wave arrives at the protection... At that time, its waveform and protection The recorded traveling wave differences are significant, such as Figure 2 As shown in (a); while in a boundaryless power grid, when the reverse traveling wave reaches the protection At that time, its waveform and protection The recorded waveforms are very similar, such as Figure 2 As shown in (b) of the diagram.

[0029] The scheme of moving the current-limiting reactors from both sides of the transmission line to a centralized arrangement at the converter station outlet will cause the disappearance of the impedance discontinuities between the line and the busbar created by the previously dispersed reactors, making the impedance between the line and the busbar more continuous. Since a clear boundary cannot be formed between the line port and the busbar, the suppression effect of the current-limiting reactors on high-frequency fault components is lost. When the fault traveling wave propagates to the node, its reflection coefficient will be greatly reduced, and most of the high-frequency energy will be directly transmitted to all connected lines, significantly enhancing the transmission effect of the traveling wave. Based on the above analysis, a Peterson equivalent circuit is constructed at the MMC1 port as follows: Figure 3 As shown.

[0030] Figure 3 middle, , These represent the line-mode impedances of line 1 and line 4, respectively. The impedance corresponding to the current-limiting reactor. This is the equivalent impedance of the converter station. The positive direction is defined as the busbar to the line direction; when a fault occurs on line 1, the traveling wave... Propagated along the line to the protection measuring point And reflection and refraction occur here, producing refracted waves. It will continue to propagate forward along line 4. At this point, in the Peterson equivalent circuit, it will be represented by a voltage twice the amplitude of the incident wave. An ideal voltage source is connected in series with the internal resistance of the line-mode wave impedance of line 1 to form a Thevenin circuit. An external branch is formed by connecting the equivalent circuit of the converter station in parallel with the line-mode wave impedance of line 4. At this point, the formula for calculating the refracted wave is: (1) Since all line parameters are equal, their wave impedances are also equal at the same frequency. Therefore, the refractive index can be obtained: (2) Among them, the line mode impedance It is related to frequency The relevant quantities are also determined by the line parameters. (3) The fault traveling wave is a high-frequency component, at which point the impedance of the linear mode wave approaches a constant. Based on the relevant formulas for the refractive index, the amplitude-frequency response curve of the refractive index can be obtained as follows: Figure 4 As shown in the figure. According to the characteristic curve, in the low-frequency range... When the refractive index approaches 1, the line exhibits a strong projection effect on the low-frequency DC component; Within the frequency band, the refractive index changes with increasing frequency, resulting in attenuation of fault components at the line port; however, in higher frequency bands... The refractive index approaches 1 again, meaning that the attenuation effect of the line on the high-frequency fault component almost disappears, and the high-frequency fault component can quickly propagate to the rest of the line.

[0031] The time-frequency characteristic image of the fault traveling wave is obtained by performing HHT transformation on the fault line-mode voltage, as shown below. Figure 5 As shown. From Figure 5 As can be seen, the first fault reverse traveling wave reaches the protection measuring point 0.31ms after the fault occurs. At this time, the frequency component of the traveling wave reaches 13kHz. According to the refractive index and amplitude-frequency characteristic diagram, the refractive index of the fault traveling wave at the line boundary is close to 1, and the fault traveling wave propagates to the next line with almost no attenuation.

[0032] The feasible fault area identification process includes: In a boundaryless power grid, due to the absence of boundary elements, the refractive index at the busbar is significantly increased while the reflection index is approximately zero. Therefore, the initial traveling wave generated when a fault occurs can propagate continuously along the ring network line and eventually reach the protection measuring point. Its specific propagation path is as follows: Figure 6 As shown To protect As the subject of this study, after a fault occurs, both a forward traveling wave and a reverse traveling wave will be generated at the fault point. Since the fault is located in the positive direction region of the protection system, the reverse traveling wave... The traveling wave will first reach the protection measuring point and trigger the first voltage surge; simultaneously, the traveling wave continues to propagate along the opposite direction of the loop network, and after a specific time delay, it will reach the protection measuring point. The voltage surge arrives at the protection measuring point at the same time, causing a second voltage spike. Differences in fault location can lead to a time difference in the arrival time of the two traveling waves at the protection device. The time difference changes and is related to the distance from the fault point to the protection device; its expression is: (4) in, The length of the path from the traveling wave to the protection point. The length of the line from which the reverse traveling wave propagates to the protection point. Let be the linear mode wave velocity, approximately taken as 299 km / ms. From the above formula, it can be seen that the farther the fault location is from the protection device, the higher the velocity. The smaller the value, the greater the difference in traveling waves between faults inside and outside the area. Figure 7 As shown. Based on this characteristic, a fault area identification criterion based on the time difference of arrival of traveling waves can be constructed.

[0033] For boundaryless power grids, the time difference of arrival of traveling waves can be used to construct fault area identification criteria. However, due to limitations in actual sampling frequency and uncertainties such as measurement errors and signal interference, the precise arrival times of two traveling wave fronts are often difficult to reliably capture. Furthermore, when a fault occurs within the fault area, the time interval between the arrival of two traveling waves is relatively long. Waiting for the second traveling wave to arrive definitively before making a judgment would fail to meet the protection's speed requirements. Therefore, this embodiment adopts a fixed-time-window discrimination method: after the initial traveling wave is detected on the line, a fixed time window of a preset duration is immediately initiated. By determining whether a second traveling wave arrives within this time window, rapid and reliable identification of faults within and outside the fault area can be achieved. This method avoids the precise dependence on the arrival times of two traveling waves, shortens the protection judgment time, and thus better meets the speed requirements.

[0034] by Figure 7 Taking a fault as an example, when the line in the forward direction... When a fault occurs at the end busbar, according to the relevant data of the four-terminal transmission network in Section 1, the time difference between the arrival time of the first reverse traveling wave at the protection measuring point and the arrival time of the forward traveling wave at the protection point is... Then when the line When an intra-regional fault occurs, the arrival time difference between the two traveling waves will increase, therefore Protection within the time window The arrival of the second traveling wave cannot be detected; the opposite is true for faults outside the zone, where the time difference between the arrival of the two traveling waves is reduced. Protection within the time window The arrival of the second traveling wave can be detected, such as Figure 8 As shown.

[0035] The feasible method for unbounded power grid traveling wave protection based on the improved Teager energy operator is as follows: (1) Protection activation criteria: When a fault traveling wave propagates to the line port, it will cause a voltage drop. Based on this, a protection criterion can be constructed using the improved voltage gradient method. The voltage gradient is: (5) The triggering criterion is: (6) In the formula, To protect the line-mode voltage gradient at the measuring point at all times; for The sampled value of the line-mode voltage at that time; This is the fault initiation threshold value. Its value should be greater than the maximum voltage gradient that occurs when the line is operating normally, and less than the minimum voltage gradient that may occur under the expected fault scenario. It is set to 25kV.

[0036] (2) Fault direction determination criteria: When a fault occurs in the forward direction, the reverse traveling wave propagates to the protection measuring point first, and no forward traveling wave is detected at this time; when a fault occurs in the reverse direction, the forward traveling wave propagates to the protection measuring point first, and no reverse traveling wave is detected at this time. Therefore, the fault direction can be determined based on the transient energy of different traveling waves. First, the transient energy of the traveling wave is defined as: (7) In the formula For the transient energy of the traveling wave, For the transient energy of the anti-traveling wave, , These represent the line-mode voltage and current fault components of the line, respectively. Let be the line-mode wave impedance of the line, then the transient energy ratio of the traveling wave is: (8) The orientation recognition criterion is set as follows: (9) In the formula The threshold value for direction identification is set to 10 because the transient energy difference between the reverse traveling wave and the protection start-up time is relatively large. After the protection starts, the sampled values ​​of the first two moments are taken to determine the fault direction. If the direction identification criteria are met in both moments, it is determined to be a positive direction fault.

[0037] (3) Criteria for determining whether a region is within or outside the region: The proposed protection principle is based on traveling wave propagation characteristics, whose speed is unaffected by transition resistance, thus effectively identifying the fault region even in the event of a high-resistance fault. However, the voltage drop amplitude caused by a high-resistance fault is relatively small, easily leading to the Teager energy value falling below the threshold set for metallic faults, causing protection failure. Therefore, this embodiment normalizes the fault line-mode voltage data, mapping it to a unified interval through linear transformation to improve the protection reliability under high-resistance faults. For a set of data, the formula for transforming it to a given interval through normalization is defined as: (10) in , These are the maximum and minimum values ​​in the data, respectively. , To map the target interval Normalization eliminates amplitude differences under different fault conditions, allowing analysis to focus more on waveform morphology and thus improving the protection system's ability to identify high-impedance faults. To avoid excessive amplification of minute fluctuations during normalization, this embodiment employs an adaptive normalization strategy: when the data range is greater than 10, it is mapped to... Interval; when the range is less than or equal to 10, it is mapped to This method enhances adaptability to high-resistivity faults while reducing the risk of misjudgment caused by noise and minor fluctuations.

[0038] For traveling wave arrival detection, an improved Teager energy operator method is used to construct the identification criterion: (11) (12) in, for The maximum value of the Teager energy was measured within the protection time. The first two data points are used for fault direction identification, and the remaining data are used for Teager energy calculation. The threshold value setting follows the principle of "retaining a large reliability margin inside the zone and an appropriate sensitivity margin outside the zone", giving priority to avoiding faults inside the zone being misjudged as faults outside the zone, while also taking into account the reliability of fault identification outside the zone.

[0039] Time window Due to sampling frequency limitations, the actual setting value has a slight rounding error compared to the theoretical value. Boundary faults within the zone are typically the most unfavorable operating conditions, with a large Teager energy value. Therefore, during engineering setting, the maximum Teager energy value when there is no voltage surge within the zone can be used as the energy reference value. This is typically obtained near the boundary fault, and a reliability factor is set. Taking line 1 as an example, the maximum Teager energy value for boundary faults within the zone is 4547.5. A reliability coefficient is selected based on a combination of energy values ​​within and outside the zone. The result of equation (12) is rounded down to the nearest integer, and a threshold value is finally set. .

[0040] (4) Fault polarity selection criterion: Once a fault is determined to be within the designated area, the fault pole needs to be identified to clear the fault. The pole selection criterion can be implemented using zero-mode voltage. Zero-mode voltage exhibits different characteristics under different fault types: it is negative for positive pole faults, positive for negative pole faults, and almost non-existent for bipolar faults. Therefore, X is defined as the average zero-mode fault voltage within the time window: (13) The fault polarity selection criterion is: (14) In the formula, is the zero-mode voltage of the fault within the time window; N is the number of sampling points within the time window, where the first two sampling points used for fault direction determination need to be deleted. The threshold value for the pole selection criterion needs to be considered to avoid the maximum unbalanced voltage when a bipolar fault occurs in the DC line of the flexible DC power grid. Considering the reliability of the criterion and reserving a certain margin, it is taken as 60kV.

[0041] (5) Criteria for lightning strike interference protection: DC transmission lines may be affected by lightning strikes during operation, which will also generate significant high-frequency transient components at the measuring points, thus interfering with protection criteria. To avoid protection malfunctions caused by lightning strikes, it is necessary to analyze the differences in transient characteristics between lightning strike disturbances and short-circuit faults, and construct corresponding lightning strike identification criteria.

[0042] Compared to short-circuit faults, lightning disturbances are characterized by rapid voltage rise, short duration, and rapid decay, typically manifesting as short-duration impulse disturbances with significant amplitude differences between the preceding and following periods. In contrast, short-circuit faults result in a rapid voltage drop and a sustained low level, exhibiting strong persistence. Therefore, this embodiment extracts two equal-length data segments within a fixed time window for comparison, and constructs the following lightning interference identification criteria based on this: (15) In the formula for The first half of the time window, The reliability coefficient for the lightning strike criterion is set to 1.3 in this embodiment. This represents the voltage amplitude ratio under fault conditions at the end of the line.

[0043] (6) Methods for eliminating dead zones: In a borderless power grid structure, due to the lack of boundary elements found in traditional transmission systems, the electrical characteristics of faults at the end of a line are similar to those at the beginning of a fault on an adjacent line outside the designated zone. Furthermore, due to inherent limitations of single-ended quantity protection, a protection dead zone forms near the line end. To address this dead zone problem, this embodiment utilizes the characteristics of under-component tripping protection, designing its operating logic as follows: When the protection on this side detects a fault within the zone in the forward direction, it immediately trips and simultaneously sends a tripping signal to the protection on the opposite side. If the protection on the opposite side determines that the fault is outside the zone and receives the tripping signal, it immediately trips as well. Figure 9 Taking the fault shown as an example, when the fault occurs in the protection When the dead zone is within range, The fault is determined to be an external fault and the protection is locked. The fault was determined to be within the area, and immediate action was taken and reported. Send trip signal; when Upon receiving a trip signal, the circuit breaker immediately trips, effectively resolving the dead zone problem.

[0044] In summary, the overall protection scheme proposed in this embodiment is as follows: Figure 10As shown. After a line fault, the protection starts when the line mode voltage meets the start criterion. First, lightning interference is identified according to equation (15). If the voltage amplitude ratio meets the lightning criterion, it is determined to be lightning interference; otherwise, it enters the direction discrimination. Then, the initial two sampling points in the time window are extracted and the fault direction is determined by equation (13). If it is a forward fault, the fault area is further identified according to equation (11). If the fault is identified as an intra-area fault, the protection immediately trips. If it is identified as an extra-area fault, but a trip signal is received from the protection on the other side, the protection also immediately operates and performs the fault pole selection process according to equation (14). Otherwise, the protection is blocked and the trip command is not executed.

[0045] This embodiment acquires voltage signals by installing measuring devices on both sides of the line, and constructs a fault criterion for line-mode voltage surge energy based on an improved Teager energy operator. It achieves a combination of fast single-ended main protection and auxiliary backup protection, further improving fault clearing capability across the entire line length while ensuring the speed and reliability of the main protection. This provides a new solution for the protection of boundaryless multi-terminal flexible DC transmission lines. This method has the following significant advantages: High speed and accuracy: Based on the characteristics of the boundaryless network structure, this embodiment studies the propagation characteristics of fault traveling waves in the network and uses an improved Teager energy operator for fault identification, resulting in significant feature differences. The fixed-time-window discrimination method avoids precise dependence on the arrival time of the traveling waves, effectively shortening the protection judgment time and better meeting the speed requirements, ensuring the speed and accuracy of fault identification.

[0046] Only voltage measurement required, simple equipment: This embodiment only requires voltage measurement for fault diagnosis, eliminating the need for complex signal measurement equipment and simplifying the system structure. By utilizing the energy difference between the preceding and following traveling waves on the line to determine the fault direction, and combining this with the improved Teager energy operator difference based on the fault line mode voltage to distinguish the fault area, the fault diagnosis process is further simplified.

[0047] Wide protection range and full-line operation: This embodiment combines single-ended main protection with auxiliary criteria. For most faults within the zone, the protection can complete the fault identification using only the traveling wave information on its own side. As a fast main protection, it can achieve instantaneous protection of more than 98% of the line range. It has the characteristics of simple structure, fast action and strong independence. For a small number of dead zone faults at the end of the line, a dual-ended auxiliary criterion based on the fast trip signal on the opposite side is introduced to supplement it. While ensuring the fast action of single-ended quantitative protection, full-range protection of the transmission line is realized.

[0048] High reliability and anti-interference capability: This embodiment can accurately identify faults under different fault locations and types, demonstrating high reliability. The improved Teager energy operator method employed has high anti-interference capability, can withstand high transition resistance and 10dB of Gaussian white noise interference, and is highly adaptable, capable of handling complex actual working conditions.

[0049] Broad Engineering Application Prospects: This embodiment is applicable to boundless multi-terminal flexible DC transmission systems, is easy to implement in practical engineering, and has broad application prospects. Its simple equipment, low sampling frequency requirements, and strong anti-interference capabilities make it highly practical in complex power systems.

[0050] In summary, this invention can effectively solve the problem of difficulty in identifying fault areas in boundaryless multi-terminal flexible DC transmission systems due to weakened boundary effects. It also has the advantages of rapid identification, wide protection range, and strong anti-interference ability, and has high engineering application value.

[0051] The feasible process of verifying and analyzing the method proposed in this embodiment by constructing a simulation model includes: A boundaryless ±500kV four-terminal flexible DC grid simulation model was built in PSCAD / EMTDC, such as... Figure 11 As shown in the figure. The sampling frequency is 100kHz. The four lines are referenced from the Zhangbei four-terminal flexible DC power grid. The lengths of l1, l2, l3, and l4 are 184.4, 101, 131, and 78km, respectively. , , , For DC bus, converter stations 1, 2, and 4 adopt constant active power control and constant reactive power control, while converter station 3 adopts constant DC voltage control and constant reactive power control.

[0052] This embodiment is for protection Taking this as an example, we will conduct a performance evaluation of the proposed protection scheme and set up... , , Distance protection The fault occurred at 3.4km, 94.4km, and 181.4km. , The fault is on the forward and reverse busbars of line 1. A fault occurred 6km outside the positive direction zone. The fault was set to occur at t=1s, with a data time window. It takes 0.42ms.

[0053] Simulations were performed to verify different fault locations and transition resistances, and the results are shown in Tables 1 and 2.

[0054] Table 1 Table 2 As shown in Tables 1 and 2, this protection method can reliably identify faults under different operating conditions. Even under extreme fault conditions at the end of the line within the zone, the protection can still withstand [faults]. The protection system is not affected by transition resistance and has strong transition resistance tolerance. Furthermore, the Teager energy values ​​corresponding to faults inside and outside the protection zone differ significantly, ensuring reliable blocking under fault conditions outside the protection zone and effectively preventing maloperation.

[0055] Even in a noisy environment where the signal-to-noise ratio drops to 10dB, the protection method proposed in this embodiment can still accurately identify the fault area, indicating that the method has good noise tolerance.

[0056] Regarding protection Fault simulation was set up at the end of the line, and the relevant results are shown in Table 3.

[0057] Table 3 As shown in Table 3, when a fault occurs 2.5 km from the end of line 1, the protection... It can still be correctly identified. At this time, the fault traveling wave propagates along the transmission line for 0.61ms. The algorithm mainly considers the time window length (0.42ms), so the protection action time is 1.03ms.

[0058] When the fault location is 2km or closer to the end, the protection Misjudgments may occur, in which case protection is necessary. The auxiliary criterion is used to implement the action. The maximum action delay caused by the auxiliary criterion is: (16) in, For maximum action delay, For line length, For the protection scope of the head end, For fiber channel transmission delay, it is usually taken as When a fault occurs 2km from the end of the line, the fault traveling wave passes through... Arrival at protection And start, protect The fault is identified and activated within the set 0.42ms time window, and a trip signal is simultaneously sent to the protection on the opposite side. The signal reaches the protection after 0.903ms of transmission via fiber optic channel. Ultimate protection The action took approximately 1.33ms.

[0059] In summary, when a fault occurs within the protection range of the line, the protection... The longest operating time is 1.03ms after the fault occurs; considering the worst-case scenario of the dead zone at the end, the total operating time of the protection is about 1.33ms, which can meet the speed requirement of traveling wave protection.

[0060] Table 4 According to the comparison results in Table 4, when the fault data is normalized and mapped to the same numerical range, the TKEO method loses its effective identification capability and cannot identify the fault area. Although the standard TEO method can distinguish between faults inside and outside the area, its anti-interference performance is weak. When the signal-to-noise ratio drops to 10dB, the area identification error occurs, leading to the protection refusing to operate. In contrast, the improved TEO method performs better in both transition resistance and noise resistance. It can accurately identify the fault area under different fault conditions, and the calculation results for faults inside and outside the area are significantly different, which is conducive to setting a more reliable threshold and further reducing the risk of misjudgment by the protection.

[0061] On the other hand, this embodiment also provides an electronic device, including a memory, a processor, and a computing program stored in the memory and executable on the processor, wherein the processor implements the method when executing the computing program.

[0062] On the other hand, this embodiment also provides a computer-readable storage medium storing a computer program that, when executed by a processor, implements the method.

[0063] The above are merely preferred embodiments of this application, but the scope of protection of this application is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in this application should be included within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of the claims.

Claims

1. A boundaryless power grid protection method based on an improved Teager energy operator, characterized in that, Includes the following steps: Obtain the line-mode voltage and line-mode current at the protection measuring point; The protection is activated based on whether the line-mode voltage gradient exceeds the activation threshold value. In response to protection activation, based on the line-mode voltage, line-mode current, and line-mode impedance collected before and after activation, the forward traveling wave transient energy and the reverse traveling wave transient energy are calculated, and it is determined whether it is a positive direction fault based on whether the ratio of the forward traveling wave transient energy to the reverse traveling wave transient energy is greater than the direction identification threshold. When a positive fault is determined, the line mode voltage is normalized, and the first two sampling points used for fault direction determination are removed within a fixed time window. The Teager energy value is calculated for each remaining sampling point after removal, and the maximum value among all Teager energy values ​​is taken as the maximum energy value of the line mode voltage. Based on the comparison between the maximum line mode voltage energy and the energy threshold value, it is determined whether it is an in-zone fault or an out-of-zone fault; When a fault is determined to be within the zone, the protection system trips; when a fault is determined to be outside the zone, the protection system is locked.

2. The method according to claim 1, characterized in that, The process of initiating protection based on whether the line-mode voltage gradient exceeds the activation threshold includes: Calculate the absolute value of the difference between the line-mode voltage at the current sampling time and the line-mode voltage at the previous sampling time, and use the obtained absolute value of the difference as the line-mode voltage gradient; when the line-mode voltage gradient is greater than or equal to the preset voltage gradient start threshold value, the protection is determined to start.

3. The method according to claim 1, characterized in that, The process of calculating the transient energy of the traveling wave and the transient energy of the anti-traveling wave includes: The instantaneous values ​​of the forward traveling wave and the reverse traveling wave are calculated based on the fault components of the line-mode voltage and the line-mode current, as well as the line-mode wave impedance. Within two sampling points after the start-up time, the sum of the squares of the instantaneous values ​​of the forward traveling wave is calculated as the transient energy of the forward traveling wave, and the sum of the squares of the instantaneous values ​​of the reverse traveling wave is calculated as the transient energy of the reverse traveling wave. The ratio of the transient energy of the forward traveling wave to the transient energy of the reverse traveling wave is calculated. When this ratio is greater than the direction recognition threshold, it is determined to be a positive direction fault.

4. The method according to claim 1, characterized in that, The process of normalizing line-mode voltage includes: Determine the maximum and minimum values ​​in the line-mode voltage data; calculate the difference between the maximum and minimum values ​​as the range; when the range is greater than a preset range threshold, linearly map the data to a first interval with a first positive number as the lower limit and a second positive number as the upper limit; when the range is less than or equal to the preset range threshold, linearly map the data to a second interval with the first positive number as the lower limit and a third positive number as the upper limit, where the second positive number is greater than the third positive number.

5. The method according to claim 1, characterized in that, The process of determining whether a fault is within or outside the zone based on the comparison between the maximum line-mode voltage energy value and the energy threshold value includes: The maximum line-mode voltage energy is compared with the energy threshold value; when the maximum line-mode voltage energy is greater than the energy threshold value, it is determined to be an external fault; when the maximum line-mode voltage energy is less than the energy threshold value, it is determined to be an internal fault.

6. The method according to claim 5, characterized in that, Once the fault is determined to be within the zone, the process also includes fault polarity selection: Once a fault is determined to be within the zone, the sampled value of the zero-mode voltage within a fixed time window is obtained; the first two sampling points used for fault direction determination are deleted. Calculate the average zero-mode voltage of the remaining sampling points; when the average zero-mode voltage is less than the negative pole selection threshold, it is determined to be a positive pole fault; when the average zero-mode voltage is greater than the positive pole selection threshold, it is determined to be a negative pole fault; when the average zero-mode voltage is between the negative pole selection threshold and the positive pole selection threshold, it is determined to be a bipolar fault.

7. The method according to claim 1, characterized in that, It also includes a lightning interference detection step, the process of which includes: After protection is activated but before direction determination, the absolute values ​​of line-mode voltage fault components within a fixed time window are extracted. The fixed time window is divided into a first half and a second half. The sum of the absolute values ​​of line-mode voltage fault components at each sampling point within the first half and the sum of the absolute values ​​of line-mode voltage fault components at each sampling point within the second half are calculated. The ratio of the sum of the first half and the sum of the second half is calculated. When this ratio is greater than the product of the preset lightning strike criterion reliability coefficient and the ratio of the fault voltage amplitude at the end of the line, it is determined to be lightning interference and the protection is blocked.

8. The method according to claim 1, characterized in that, It also includes a protection dead zone elimination step, the process of which includes: When the protection on this side determines that the fault is outside the positive direction zone, the protection on this side is locked and does not operate; when the protection on this side receives a trip signal sent by the protection on the other side, it immediately performs the trip operation; the condition for the protection on the other side to send a trip signal is that the protection on the other side determines that the fault is inside the zone and performs the trip operation.

9. An electronic device comprising a memory, a processor, and a computing program stored in the memory and executable on the processor, characterized in that, When the processor executes the computing program, it implements the method of any one of claims 1-8.

10. A computer-readable storage medium storing a computer program, characterized in that, When the computer program is executed by a processor, it implements the method of any one of claims 1-8.

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