An adaptive time window line differential protection method based on transient band energy features

Abstract

The application discloses a kind of adaptive time window line differential protection methods based on transient band energy characteristics, it is related to electric power system relay protection technical field.The method first collects the differential current of the line to be protected both ends, whether the line is judged to occur fault based on differential current;Judgment occurs after the construction transient analysis time window, the differential current in time window is carried out band decomposition, obtain low-frequency action energy and high-frequency brake energy, and calculate the energy ratio of two;Whether to execute in-zone fault protection action is judged based on low-frequency action energy and energy ratio;If it is judged not to execute in-zone fault protection action, then the length of transient analysis time window is extended, and low-frequency action energy and energy ratio are updated based on the time window after extension, and in-zone fault protection action is judged again.The application is balanced by the iterative discrimination mechanism of time window length incremental step by step, the speed of transient quantity protection and action reliability, improves the discrimination sensitivity under weak fault scene.

Description

Technical Field

[0001] This application relates to the field of power system relay protection technology, specifically to an adaptive time-window line differential protection method based on transient frequency band energy characteristics. Background Technology

[0002] With the high proportion of new energy sources integrated into the power system, the complexity and dynamism of power grid operation have significantly increased. The non-stationary characteristics of transient signals from transmission line faults have become increasingly prominent, placing higher demands on the speed and reliability of line protection. Transient differential protection, based on the transient electrical quantities generated by faults to construct differential criteria, has advantages such as fast operating speed, immunity to system oscillations, and strong resistance to current transformer saturation. It has become an important technical direction in the field of high-voltage and ultra-high-voltage transmission line protection.

[0003] Current transient differential protection schemes generally employ fixed-length analysis windows to capture fault transient signals, extracting characteristic quantities and then performing fault identification within / outside the protection zone in a single operation. This approach presents an inherent technical contradiction: while short analysis windows can effectively shorten protection action delay and improve speed, the transient characteristics of the fault within the short window are not fully developed, leading to insufficient characteristic quantities and protection failure in weak fault scenarios such as high-resistance grounding within the protection zone. Simultaneously, the high-frequency transient components generated by faults outside the protection zone have not sufficiently attenuated, easily causing maloperation and overall insufficient reliability. Conversely, long analysis windows can obtain more complete transient characteristic information and improve fault identification accuracy, but the protection action delay increases significantly, failing to meet the stringent speed requirements of high-voltage transmission lines.

[0004] Application content

[0005] To overcome the shortcomings of existing technologies, this application provides an adaptive time-window line differential protection method based on transient frequency band energy characteristics. Through an iterative discrimination mechanism with progressively increasing time window length, and combined with dual criteria of low-frequency action energy and energy ratio corresponding to each time window length, progressive fault discrimination is performed. This solves the technical problems of existing fixed-duration sliding time window schemes, which cannot balance protection speed and action reliability, fixed setting value system, which cannot adapt to the dynamic change law of transient energy over time window span, and insufficient discrimination sensitivity in weak fault scenarios.

[0006] The adaptive time-window line differential protection method based on transient frequency band energy characteristics provided in this application specifically adopts the following technical solution: An adaptive time-window line differential protection method based on transient frequency band energy characteristics includes: acquiring the differential current at both ends of the protected line and determining whether a fault has occurred based on the differential current; constructing a transient analysis time window when a fault is determined to have occurred, performing frequency band decomposition on the differential current within the transient analysis time window to obtain low-frequency operating energy and high-frequency braking energy, and calculating the ratio of the low-frequency operating energy to the high-frequency braking energy to obtain an energy ratio; determining whether to execute an intra-zone fault protection action based on the low-frequency operating energy and the energy ratio; extending the length of the transient analysis time window when it is determined not to execute the intra-zone fault protection action; updating the low-frequency operating energy and the energy ratio based on the extended transient analysis time window, and determining whether to execute the intra-zone fault protection action based on the updated low-frequency operating energy and the energy ratio.

[0007] In some embodiments of this application, the method for extending the length of the transient analysis window includes: presetting an incremental length and a window length threshold; the transient analysis window having an initial length; when it is determined that no fault protection action within the area will be performed, and the length of the transient analysis window is less than the window length threshold, the incremental length is increased based on the initial length to extend the transient analysis window.

[0008] In some embodiments of this application, the method for extending the length of the transient analysis window further includes: when a fault is determined to occur, determining the fault start time based on the differential current; using the fault start time as the starting point of the transient analysis window, extending the end point of the transient analysis window based on the incremental length.

[0009] In some embodiments of this application, the method further includes: when it is determined that the fault is an external fault, and the length of the transient analysis window is extended to a value not less than the window length threshold, the fault is determined to be an external fault.

[0010] In some embodiments of this application, the method for determining whether to perform an intra-zone fault protection action includes: calculating the ratio of the maximum low-frequency energy to the maximum energy among various types of extra-zone faults within the transient analysis time window; setting a reliability coefficient; multiplying the maximum low-frequency energy by the reliability coefficient to obtain a low-frequency energy setting value, and multiplying the maximum energy ratio by the reliability coefficient to obtain an energy ratio setting value; determining whether the low-frequency energy is greater than the low-frequency energy setting value and whether the energy ratio is greater than the energy ratio setting value within the transient analysis time window; determining that the fault is an intra-zone fault when the low-frequency energy is greater than the low-frequency energy setting value and the energy ratio is greater than the energy ratio setting value; and performing an intra-zone fault protection action when the fault is determined to be an intra-zone fault.

[0011] In some embodiments of this application, the method further includes: determining the length of all constructable transient analysis windows based on the initial length, incremental length, and window length threshold; calculating the low-frequency energy setting value and the energy ratio setting value within each transient analysis window; constructing a setting value lookup table based on the length of each transient analysis window and the low-frequency energy setting value and the energy ratio setting value within each transient analysis window; and, when determining whether to perform an intra-area fault protection action, calling the low-frequency energy setting value and the energy ratio setting value corresponding to the length of the transient analysis window in the setting value lookup table according to the currently constructed transient analysis window.

[0012] In some embodiments of this application, the method for frequency band decomposition of the differential current within the transient analysis window includes: dividing the differential current within the transient analysis window into frequency bands to obtain a low-frequency action component and a high-frequency braking component; calculating the energy of the low-frequency action component within the transient analysis window to obtain the low-frequency action energy; and calculating the energy of the high-frequency braking component within the transient analysis window to obtain the high-frequency braking energy.

[0013] In some embodiments of this application, the method for band division of the differential current within the transient analysis window includes: decomposing the differential current within the transient analysis window using a complete ensemble empirical mode decomposition algorithm to obtain multi-order intrinsic mode function components and residual components; calculating the zero-crossing rate of each order of the intrinsic mode function components within the current transient analysis window, and converting the equivalent frequency of each order of the intrinsic mode function components based on the zero-crossing rate; preset a frequency boundary value; classifying the intrinsic mode function components whose equivalent frequency is not greater than the frequency boundary value into low-frequency action components, classifying the intrinsic mode function components whose equivalent frequency is greater than the frequency boundary value into high-frequency braking components, and classifying the residual components into low-frequency action components.

[0014] In some embodiments of this application, the method for determining whether a fault has occurred includes: setting a preset current surge threshold; acquiring the differential current at each sampling time; calculating the difference between the differential current at the current sampling time and the differential current at the corresponding sampling time of the previous power frequency cycle, and calculating the magnitude of the difference to obtain the differential current surge threshold; determining whether the differential current surge threshold is greater than the current surge threshold; and if the differential current surge threshold is greater than the current surge threshold, then determining that a fault has occurred.

[0015] In some embodiments of this application, the method for determining whether a fault has occurred further includes: setting a preset zero-sequence voltage mutation start value; acquiring the zero-sequence voltage at both ends of the protected line at each sampling time; calculating the difference between the zero-sequence voltage at the current sampling time and the zero-sequence voltage at the corresponding sampling time of the previous power frequency cycle, and calculating the magnitude of the difference to obtain the zero-sequence voltage mutation amount; determining whether the zero-sequence voltage mutation amount is greater than the zero-sequence voltage mutation start value; if the zero-sequence voltage mutation amount is greater than the zero-sequence voltage mutation start value, then determining that a fault has occurred.

[0016] The technical solution of this application achieves the following beneficial effects: 1. A transient analysis time window with a fixed starting point and progressively increasing length and an iterative discrimination mechanism are adopted. When the fault characteristics are obvious, the short time window is used to act quickly to ensure the protection speed. In the case of weak faults, the time window is gradually extended to accumulate transient energy characteristics to improve the discrimination accuracy. This fundamentally balances the inherent contradiction between protection speed and action reliability.

[0017] 2. Construct a dynamic tuning value system that matches the time window length of each transient analysis level. Based on the reliability coefficient, quantify the action threshold under each window length. With the help of the setting value reference table, realize offline tuning and online calling. The tuning value and the transient energy change law with the window length are accurately matched, which not only ensures that the fault outside the zone does not reliably take action, but also improves the sensitivity of weak faults within the zone.

[0018] 3. The signal processing scheme adopting adaptive decomposition combined with equivalent frequency division can adaptively match the spectral characteristics of non-stationary transient signals without relying on fixed basis functions. Compared with the traditional fixed frequency division method, it improves the extraction accuracy of high and low frequency energy and has stronger criterion stability under different fault types and transition resistance conditions.

[0019] 4. Configure dual start-up criteria for differential current surge and zero-sequence voltage surge. Differential current surge covers most fault scenarios to ensure fast operation, while zero-sequence voltage surge supplements weak feeder fault scenarios such as high-resistance grounding. The two criteria are used in conjunction with OR logic to comprehensively improve the coverage of fault start-up and avoid protection failure under weak faults. Attached Figure Description

[0020] Figure 1 This is a schematic diagram of the overall process of the adaptive time-window line differential protection method based on transient frequency band energy characteristics described in this application; Figure 2 A schematic diagram of the topology of a double-ended transmission line system to which the protection method described in this application is applied; Figure 3 This is a schematic diagram illustrating the principle of progressively increasing transient analysis time windows as described in this application. Figure 4This is a schematic diagram showing the correspondence between different length transient analysis windows and fault differential currents described in this application; Figure 5 This is a schematic diagram of the multi-order intrinsic mode function components of the differential current described in this application after complete ensemble empirical mode decomposition; Figure 6 This is a schematic diagram of the division results of the high-frequency braking component and the low-frequency action component based on zero crossover rate as described in this application; Figure 7 This is a schematic diagram illustrating the cumulative evolution characteristics of high-frequency braking energy and low-frequency action energy during the fault process described in this application. Figure 8 This is a three-dimensional motion characteristic diagram of the fault criteria in the two zones described in this application. Detailed Implementation

[0021] To make the technical problems, technical solutions, and beneficial effects to be solved by this application clearer, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and are not intended to limit the scope of this application.

[0022] The prefixes such as "first" and "second" used in this application embodiment are merely for distinguishing different descriptive objects and do not limit the position, order, priority, quantity, or content of the described objects. The use of ordinal numbers and other prefixes used to distinguish descriptive objects in this application embodiment does not constitute a limitation on the described objects. The description of the described objects is given in the claims or the context of the embodiments, and should not constitute unnecessary restrictions due to the use of such prefixes. Furthermore, in the description of this embodiment, unless otherwise stated, "multiple" means two or more.

[0023] The technical solutions of the embodiments of this application will be described below with reference to the accompanying drawings. In the description of the embodiments of this application, unless otherwise stated, " / " means "or," for example, A / B can mean A or B; the term "and / or" in this document is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, and B existing alone.

[0024] In the embodiments provided in this application, it should be understood that the disclosed systems and methods can be implemented in other ways. For example, the device embodiments described above are merely illustrative. For instance, the division of units is only a logical functional division, and in actual implementation, there may be other division methods. For example, multiple units or components may be combined or integrated into another system, or some features may be ignored or not executed. Furthermore, the coupling or direct coupling or communication connection shown or discussed may be through some interfaces; the indirect coupling or communication connection between devices or units may be electrical, mechanical, or other forms.

[0025] like Figure 2 As shown, the adaptive time-window line differential protection method based on transient frequency band energy characteristics of this application is applied to a double-ended transmission line system, where the transmission line is the protected line. The two ends of the protected line are respectively equipped with M-side busbar and N-side busbar, and current measuring devices and protection devices are installed at both busbars. The current measuring devices and protection devices cooperate through synchronous sampling and data interaction to form the execution carrier of the protection method of this application.

[0026] The protected line's M-side connects to the system side, which is connected to a synchronous power frequency AC power supply. The protected line's N-side connects to the wind power side via a main transformer. The wind power side includes a wind power grid-connected converter, corresponding to a grid operation scenario with a high proportion of renewable energy integration. The main transformer adopts a Yd connection configuration to achieve voltage matching and electrical isolation between the wind power side and the transmission line.

[0027] The current i on the M side was collected at the line outlet of the M side bus. M The N-side current i is collected at the line outlet of the N-side bus. N The protection device calculates the differential current of the protected line by synchronously exchanging current data on both sides, and uses this as the basic electrical quantity for deciding whether to perform fault protection.

[0028] Fault points f1 to f5, set at different locations along the protected line, are used to distinguish the fault range: Fault points f2, f3, and f4 are located within the transmission line range between the M-side bus and the N-side bus, and belong to the in-zone faults described in the protection method of this application. When the protection device determines that it is an in-zone fault, it should perform a tripping action; Fault point f1 is located on the system side outside the M-side bus, and fault point f5 is located on the main transformer and wind power side outside the N-side bus. Both belong to the out-of-zone faults described in the protection method of this application. When it is determined that it is an out-of-zone fault, the protection device should reliably lock out and not perform the in-zone fault protection action.

[0029] This embodiment uses a sampling frequency of 10kHz to discretely sample the electrical quantities of the transmission line. The time point corresponding to each sampling point is the sampling time, and the sampling interval is 0.1ms. The standard power frequency of my country's power grid is 50Hz, corresponding to a power frequency period of 20ms. Therefore, one power frequency period contains 200 sampling points. The positive direction of current is defined as the flow from the bus to the line. Let the sampling value of the current of the same phase on the M side at the k-th sampling time be i. M (k), the sampled value of the phase current of the same name on the N side is i N (k), then the corresponding differential current sampling value is i d (k)=i M (k)+i N (k). The zero-sequence voltage is calculated from the three-phase bus voltage, or it can be directly measured through the open delta winding of the voltage transformer. This protection method performs fault judgment and fault protection procedures for each of the three phases separately; the following explanations all use a single phase as an example.

[0030] The following is a detailed description of the complete execution flow of the adaptive time-window line differential protection method based on transient frequency band energy characteristics in this embodiment, such as... Figure 1 As shown, the specific steps are as follows: S1. Fault Initiation Judgment and Fault Start Time Determination The protection device collects electrical quantities at both ends of the protected line in real time, calculates the differential current based on the collected electrical quantities, and determines whether a fault has occurred in the protected line based on the differential current.

[0031] This step employs a fault diagnosis architecture that combines dual criteria or logic, including judgment based on differential current fluctuations and judgment based on zero-sequence voltage fluctuations, comprehensively covering the startup requirements of different fault types.

[0032] 1.1 Judgment of differential current sudden change The current surge trigger value is preset and is adjusted offline according to the principle of avoiding the maximum unbalanced current and measurement noise under normal operating conditions. The adjustment process covers operating conditions such as normal operation, load fluctuation and measurement noise, and reserves sufficient reliability margin to ensure that it will not start falsely under normal operating conditions.

[0033] The protection device acquires differential current sample values ​​at each sampling time, and sets the differential current sample value i at the current k-th sampling time. d (k), the differential current sample value i at the sampling time corresponding to the previous power frequency cycle. d By subtracting the two values ​​(kN), the magnitude of the difference is calculated to obtain the differential current surge at time k. Where N is the number of sampling points corresponding to one power frequency cycle.

[0034] The calculated differential current surge ΔI(k) is compared with the preset current surge trigger value. When ΔI(k) > ΔI set When this occurs, it is determined that the protected line has a fault or strong disturbance.

[0035] This differential current mutation judgment method has a fast response speed for faults with obvious characteristics such as metallic short circuits and low resistance faults, and can be used as a core fault criterion.

[0036] 1.2 Judgment of Zero-Sequence Voltage Sudden Change The zero-sequence voltage mutation start-up value is preset. The zero-sequence voltage mutation start-up value is set offline according to the principle of avoiding the maximum unbalance fluctuation of zero-sequence voltage under normal operating conditions, and the necessary reliability margin is reserved to avoid false start-up under normal operating conditions.

[0037] The protection device acquires the zero-sequence voltage sample value at each sampling time. It then subtracts the zero-sequence voltage sample value U0(k) at the current k-th sampling time from the zero-sequence voltage sample value U0(kN) at the corresponding sampling time of the previous power frequency cycle. The magnitude of this difference is then calculated to obtain the zero-sequence voltage abrupt change at the k-th time. Where N is the number of sampling points corresponding to one power frequency cycle.

[0038] The calculated zero-sequence voltage mutation is compared with the preset zero-sequence voltage mutation initiation value. When ΔU0(k) > ΔU 0set At that time, it is determined that there is a fault in the protected line.

[0039] This zero-sequence voltage mutation judgment method is highly sensitive to high-resistance grounding faults and asymmetrical grounding faults, and is used to supplement the insufficient sensitivity of differential current mutation judgment in weak feeder fault scenarios.

[0040] 1.3 Determining the Fault Initiation Time When the above ΔI(k) > ΔI set and ΔU0(k)>ΔU 0set If any one of the conditions is met, the protection device determines that there is a fault in the protected line. It will then initially determine that ΔI(k) > ΔI. set Or ΔU0(k)>ΔU 0set When the sampling time is determined, the corresponding sampling time is defined as the fault initiation time t0. If multiple phases of the protected line simultaneously satisfy ΔI(k)>ΔI set Or ΔU0(k)>ΔU 0set The earliest detected sampling time is taken as the unified fault start time to ensure the consistency of the time window truncation for each phase.

[0041] S2. Transient analysis time window construction and intra-region fault iteration judgment After determining that a fault exists in the protected line, a transient analysis window is constructed with the fault initiation time t0 as the starting point to capture the transient fault signal and extract feature quantities. This embodiment adopts a sliding extended window architecture with a fixed starting point and a gradually extending ending point, which compensates for insufficient feature extraction in weak fault scenarios while ensuring rapid fault protection action.

[0042] The transient analysis window has an initial length, which is 5ms in this embodiment, including 50 sampling points. The initial transient analysis window range is [t0, t0+5ms]. The initial transient analysis window is set to 5ms. On the one hand, this covers the initial transient phase after a fault occurs, meeting the need for rapid identification of faults in most obvious areas; on the other hand, the current transformer has not yet entered saturation during this time, ensuring high accuracy of the sampled data and avoiding measurement errors caused by current saturation.

[0043] Furthermore, preset thresholds for increment length and time window length are defined. The increment length is set to 1ms, including 10 sampling points. In engineering, it can also be set to a number of integer sampling points according to the sampling frequency. The time window length threshold is the maximum length of the transient analysis time window, set to 10ms, including 100 sampling points. The maximum length of the transient analysis time window is still less than half a power frequency cycle. Under the premise of appropriately sacrificing a very small amount of speed, the weak fault identification capability is significantly improved, while still meeting the engineering requirements for rapid fault clearing of transient protection.

[0044] After a fault is detected, it is determined whether to execute the fault protection action within the zone. If the current transient analysis window determines that the fault protection action within the zone should not be executed, and the length of the current transient analysis window is less than the window length threshold, then the length of the current transient analysis window is extended: the fault start time t0 is kept fixed as the starting point of the transient analysis window, and the end point of the initial length of the transient analysis window is extended by an incremental length to form the extended transient analysis window.

[0045] like Figure 3 and Figure 4 As shown, with the fault initiation time as a fixed left boundary, the transient analysis window starts from an initial length and increases in length by successively extending to the right towards the endpoint, intuitively demonstrating the iterative mechanism of a fixed starting point and progressively extended endpoint. The lengths of all constructable transient analysis windows are 5ms, 6ms, 7ms, 8ms, 9ms, and 10ms, respectively. The longer the transient analysis window, the larger the range of transient data of the protected line it covers. For each constructed transient analysis window, frequency band decomposition, feature extraction, and fault judgment are re-executed based on the complete differential current data within that window.

[0046] The transient analysis window of this scheme does not shift along the time axis as a whole. Each calculation includes the initial mutation information of the fault. This can not only gradually absorb the cumulative effect of subsequent low-frequency components and compensate for the problem of insufficient low-frequency energy extraction in short time windows under unfavorable fault initial phase angle, but also completely retain the high-frequency braking information in the initial stage of the fault, avoiding the reduction of blocking reliability due to the loss of initial high-frequency characteristics when there is a fault outside the zone.

[0047] S3. Differential Current Frequency Band Decomposition and Energy Calculation The differential current within the current transient analysis window is decomposed into frequency bands to obtain the low-frequency action component and the high-frequency braking component. The energy of the two components within the current transient analysis window is calculated to obtain the low-frequency action energy and the high-frequency braking energy, and the energy ratio between the two is calculated.

[0048] Frequency band division can be achieved using various general signal processing methods such as wavelet transform and empirical mode decomposition. The low-frequency action energy and high-frequency braking energy are calculated by integrating the square of the instantaneous value of the differential current within the transient analysis window. As a preferred implementation, this embodiment employs a Complete Ensemble Empirical Mode Decomposition (CEEMD) algorithm combined with a zero-crossing-rate adaptive frequency division scheme to improve the processing accuracy of non-stationary transient signals. Specifically, this includes: 3.1 Complete Set Empirical Mode Decomposition For the length T of the current transient analysis window, the differential current sequence x(t)=id(t) within the current transient analysis window is taken as the decomposition input, and the complete set empirical mode decomposition algorithm is used for adaptive decomposition.

[0049] Traditional fixed basis function decomposition methods require pre-setting frequency band boundaries or decomposition scales, making them susceptible to the effects of sampling frequency, decomposition level, and local signal abrupt changes. They also lack adaptability to non-stationary and nonlinear fault transient signals. In contrast, the complete ensemble empirical mode decomposition algorithm does not require pre-setting fixed mother functions. It can separate intrinsic mode function components at different time scales based on the signal's local extrema and time scale characteristics, making it more suitable for the nonlinear and non-stationary characteristics of fault transient currents in renewable energy integration scenarios.

[0050] The decomposition steps are as follows: M sets of Gaussian white noise sequences with the same amplitude and opposite phase are added to the differential current sequence x(t) to obtain 2M sets of auxiliary signals; where M is the set degree of the complete set empirical mode decomposition. Empirical mode decomposition (EMD) is performed on each group of auxiliary signals to obtain the corresponding intrinsic mode function (IMF) components of each order. A ensemble average is then calculated for IMF components of the same order to eliminate the influence of additional white noise, ultimately yielding the decomposition results. ; In the formula, F k R(t) represents the k-th intrinsic mode function component, k=1,2,…,K, where K is the total order of the intrinsic mode function components; R(t) represents the residual component, which represents the slow change trend term of the signal.

[0051] like Figure 5 As shown, the differential current sequence is decomposed to obtain multi-order intrinsic mode function components and residual components. The components are arranged in descending order of oscillation frequency, which reflects the multi-scale separation effect of adaptive decomposition on non-stationary transient signals.

[0052] By superimposing paired positive and negative white noise and ensemble averaging, the effects of mode mixing and residual noise can be effectively reduced, allowing transient current components at different time scales to be separated more naturally. In engineering implementation, the amplitude of the added white noise is taken as 0.2 times the standard deviation of the original differential current, the number of ensembles M is taken as 50 to 100, and the maximum decomposition order is limited to 8 to 10. Since the longest transient analysis time window in this scheme is only 10ms, the length of the differential current data corresponding to a single decomposition is limited. The overall computational load can be constrained by the above parameters to meet the real-time requirements of the protection device.

[0053] 3.2 High- and low-frequency classification based on zero cross-linking rate For each intrinsic mode function component obtained from the decomposition, the total number of times it crosses the zero axis within the current transient analysis window is counted. The zero-crossing rate of the intrinsic mode function component is obtained by dividing the number of zero-crossings of the intrinsic mode function component by the length of the current transient analysis window. The equivalent frequency of each intrinsic mode function component is then calculated by dividing the obtained zero-crossing rate by 2.

[0054] Since the order of the intrinsic mode function (IMF) obtained from complete set empirical mode decomposition does not strictly correspond one-to-one with the actual frequency under all operating conditions, fault type, transition resistance, initial phase angle, and noise can all alter the local oscillation characteristics of each IMF component. If high and low frequencies are only classified according to component number, weak low-frequency fault components may be mistakenly classified as high-frequency braking components. This scheme uses zero crossover rate to calculate the equivalent frequency, which can more directly reflect the oscillation speed of each mode, has a small computational load, is convenient for real-time calculation by protection devices, and can adapt to changes in the number and frequency distribution of IMF components under different fault scenarios.

[0055] For the k-th order component F k Let M be the number of zero-crossings of (t) within the transient analysis time window T. k The formula for calculating the equivalent frequency is: ; In the formula, f zc,kM is the equivalent frequency of the k-th order natural mode function component; k is the number of zero-crossings of the corresponding intrinsic mode function component within the current transient analysis window; T is the length of the current transient analysis window.

[0056] A preset frequency boundary value is used as the dividing line between high and low frequencies. This frequency boundary value can be calculated based on the distributed inductance and distributed capacitance parameters of the protected line to obtain the lowest transient dominant frequency of the fault outside the protection zone, and is selected accordingly. A typical value is on the order of several hundred Hz. Under normal circumstances, during faults outside the protection zone, the dominant high-frequency component in the differential current of the protected line is generated by the charging and discharging process of the distributed capacitance and inductance of the protected line, and its frequency is higher than the power frequency. During faults inside the protection zone, the differential current of the protected line is continuously supplied by the fault-addressed power supply, and low-frequency and power frequency components dominate. Using this frequency boundary value as the dividing line allows the low-frequency action component and the high-frequency braking component to have clear physical meanings.

[0057] The classification rules are as follows: intrinsic mode function components with an equivalent frequency not greater than the frequency threshold are classified as low-frequency action components; intrinsic mode function components with an equivalent frequency greater than the frequency threshold are classified as high-frequency braking components; and residual components are classified as low-frequency action components.

[0058] like Figure 6 As shown, all intrinsic mode function components and residual components are divided into a set Ω of high-frequency braking components according to a preset frequency boundary value. H The set of low-frequency action components Ω L The two types of components correspond to braking information and motion information respectively, with clear boundaries.

[0059] 3.3 Component Reconstruction and Energy Calculation Based on the high- and low-frequency classification results, the intrinsic mode function components classified as high-frequency braking components are superimposed, and the intrinsic mode function components and residual components classified as low-frequency motion components are superimposed to reconstruct the low-frequency motion component i within the current transient analysis window. L (t) and high-frequency braking component i H (t): ; In the formula, Ω L Ω is the collection of low-frequency action components. H It is a collection of high-frequency braking components.

[0060] The energy of the low-frequency motion component and the high-frequency braking component within the current transient analysis window are calculated separately to obtain the low-frequency motion energy E. L (T) and high-frequency braking energy E H(T) is calculated by integrating the squares of the instantaneous values ​​of the low-frequency action component and the high-frequency braking component within the transient analysis window. In the discrete sampling implementation, the differential current consists of individual sampling points. Therefore, the instantaneous values ​​of each sampling point within the transient analysis window are squared and summed to calculate the energy within the corresponding transient analysis window. The calculation principle is consistent with the continuous integration method. ; In the formula, i L (n) represents the instantaneous value of the low-frequency motion component at the nth discrete sampling point; i H (n) represents the instantaneous value of the high-frequency braking component at the nth discrete sampling point; n is the index of the discrete sampling point; [t0, t0+T] is the time interval of the current transient analysis window, and the summation operation takes values ​​from all discrete sampling points within the time interval [t0, t0+T].

[0061] Among them, low-frequency operating energy is used to reflect the effective operating quantity provided by the continuous fault source of the fault within the zone in the differential current. When the fault is within the zone, the low-frequency energy in this part will continue to accumulate as the transient analysis time window is extended. High-frequency braking energy is used to reflect the impact of the charging and discharging of the distributed parameters of the protected line, the imbalance of the through current of the fault outside the zone, and the high-frequency disturbance on the differential circuit. When the fault is outside the zone, the high-frequency component is dominant and gradually decays over time.

[0062] To characterize the dominance of the low-frequency motion component relative to the high-frequency braking component, the ratio of low-frequency motion energy to high-frequency braking energy is calculated to obtain the energy ratio. The formula for calculating the energy ratio λ(T) is as follows: ; In the formula, ε is a very small positive number set to prevent the denominator from being zero, thus avoiding calculation abnormalities when the high-frequency braking energy is zero.

[0063] The energy ratio further characterizes the dominance of low-frequency action quantity relative to high-frequency braking quantity, avoiding misjudgments caused by the occasional increase of low-frequency components due to large external disturbances when relying solely on low-frequency action energy. During faults within the zone, the differential current mainly consists of power frequency and low-frequency fault components. As the transient time window lengthens, low-frequency energy continues to accumulate, and the overall energy ratio increases. During faults outside the zone, the differential current mainly consists of damped high-frequency oscillations caused by distributed parameters. The proportion of high-frequency braking energy is relatively high, and low-frequency action energy is difficult to exceed the action threshold.

[0064] like Figure 7 As shown, Figure 7(a) In the corresponding fault scenario within the zone, the left vertical axis represents the amplitude of the differential current, the right vertical axis corresponds to the energy value, and the horizontal axis represents time. After the fault occurs, the low-frequency action energy continues to rise and accumulate over time, reflecting the characteristics of the low-frequency action component continuously existing and the energy steadily increasing under fault conditions within the zone. Figure 7 (b) For fault scenarios outside the fault zone, the left vertical axis represents the amplitude of the differential current, the right vertical axis corresponds to the energy value, and the horizontal axis represents time. In the initial stage of an external fault, the high-frequency braking energy rises rapidly. Subsequently, as the high-frequency braking component gradually decays, the growth rate of the high-frequency braking energy gradually slows down and tends to stabilize, reflecting the characteristic that the high-frequency braking component dominates under external faults and gradually decays over time. The evolution patterns of high-frequency and low-frequency energy differ significantly between the two fault scenarios, providing a clear physical basis for distinguishing between faults inside and outside the fault zone.

[0065] S4. In-zone / Out-of-zone Fault Detection and Time Window Iteration Based on the low-frequency action energy and energy ratio calculated under the current transient analysis window, it is determined whether to execute the fault protection action within the zone. If it is determined not to execute the fault protection action within the zone, the length of the transient analysis window is extended. The low-frequency action energy and energy ratio are updated based on the extended transient analysis window, and it is determined whether to execute the fault protection action within the zone based on the updated low-frequency action energy and energy ratio. If the determination result is still not to execute the fault protection action within the zone, the length of the transient analysis window is extended again, and the window extension, feature update and fault discrimination process is iteratively executed until it is determined to execute the fault protection action within the zone or the length of the transient analysis window reaches the window length threshold.

[0066] For each transient analysis window, if both of the following conditions are met simultaneously, it is determined to be an intra-zone fault, and the protection device will perform a trip protection action: The low-frequency action energy is greater than the low-frequency energy setting value of the corresponding transient analysis time window, and the energy ratio is greater than the energy ratio setting value of the corresponding transient analysis time window.

[0067] The two fault criteria mentioned above employ a combination of "action quantity + ratio constraint": if only the low-frequency action energy exceeds the threshold but the energy ratio fails to meet the standard, it indicates that the high-frequency braking component is still significant and is insufficient to determine an in-zone fault; if only the energy ratio is high but the absolute value of the low-frequency action energy is insufficient, it may be an occasional fluctuation caused by high-frequency energy attenuation, lacking a basis for action. The combination of these two criteria with logic can effectively reduce the risk of erroneous operation, balancing action sensitivity and interlocking reliability. Figure 8 As shown, the action plane is constructed with low-frequency action energy and high-frequency braking energy as dimensions. The area that satisfies the fault criteria in the two zones is the action zone, and the remaining area is the non-action zone. This intuitively demonstrates the action boundary and braking characteristics of the fault criteria in the zone.

[0068] The complete discrimination and iteration process is as follows: After the protection is started, the frequency band decomposition and judgment are first performed in the initial transient analysis window of 5ms. If the fault criteria in both zones are met at the same time, it is determined to be a fault in the zone, and the protection device will immediately trip to ensure the speed of operation in scenarios with obvious fault characteristics. If it is determined that no fault protection action within the zone will be performed within the initial transient analysis window, the length of the transient analysis window will be increased by an increment of 1ms. Within the extended transient analysis window, complete set empirical mode decomposition, zero crossover rate frequency division, energy calculation and fault identification will be re-executed. When the length of the transient analysis window is extended to a window length threshold of not less than 10ms, if it is still determined that the fault protection action within the zone will not be executed, it will be determined as an external fault, external disturbance or non-fault phase, and the protection device will be reliably locked, terminating the current judgment process.

[0069] For weak faults or unfavorable initial phase angle faults within the protection zone, the low-frequency action components and aperiodic components in the initial transient analysis window may cancel each other out, leading to an underestimation of the low-frequency action energy and failure to meet the conditions for executing fault protection actions. In this case, by gradually extending the time window to supplement the low-frequency action energy, the characteristic quantities of the fault within the protection zone can be gradually raised and exceed the action threshold, avoiding the risk of failure to operate under a fixed short time window. However, faults outside the protection zone lack continuous low-frequency fault source support, and even if the high-frequency braking component decays, it is difficult to simultaneously meet the two fault criteria within the protection zone within the longest transient analysis window, thereby ensuring the reliability of the blocking.

[0070] S5. Setting Value Setting and Setting Value Comparison Table Call In this embodiment, the tuning value adopts an offline tuning and online calling mode. The tuning value is tuned separately for the length of each transient analysis window to ensure that the tuning value is accurately matched with the transient energy evolution law under the corresponding transient analysis window.

[0071] During the offline tuning phase, for each transient analysis time window length, various external fault scenarios are traversed, including different fault types, different fault locations, different transition resistances, different fault initial phase angles, and typical noise interference conditions. The ratio of the maximum low-frequency action energy to the maximum energy of the external fault under that transient analysis time window length is statistically obtained.

[0072] A preset reliability coefficient, with a value greater than 1, is used to ensure the necessary safety margin. The low-frequency energy setting value within the transient analysis window is obtained by multiplying the maximum low-frequency action energy of the fault outside the zone by the reliability coefficient; the energy ratio setting value within the transient analysis window is obtained by multiplying the maximum energy ratio of the fault outside the zone by the reliability coefficient.

[0073] Since the high-frequency braking energy of external faults decays over time, the energy ratio will dynamically change with the length of the transient analysis window. Therefore, by setting the length of each transient analysis window separately, it can be ensured that extending the length of the transient analysis window will not reduce the blocking reliability of external faults.

[0074] Based on the initial length of 5ms, the incremental length of 1ms, and the time window length threshold of 10ms, the lengths of all 6 transient analysis time windows are determined. The low-frequency energy setting value and energy ratio setting value corresponding to the length of each transient analysis time window are summarized to construct a setting value reference table with a one-to-one correspondence between "time window length and setting value", which is stored in the protection device.

[0075] During the online discrimination phase, each time the protection device switches between transient analysis time windows, it compares the corresponding low-frequency energy setting value and energy ratio setting value in the setting comparison table based on the length of the current transient analysis time window. This eliminates the need for real-time online calculation of setting values, resulting in low computational load, fast response, and suitability for actual engineering applications. If the on-site operating conditions change significantly, multiple sets of setting comparisons can be pre-set to form a setting comparison table. The corresponding low-frequency energy setting value and energy ratio setting value can be switched according to operating conditions such as line length, short-circuit capacity of the transmission line system, and renewable energy output level.

[0076] The above description is merely a specific embodiment 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 technical scope disclosed in this application should be covered. Therefore, the scope of protection of this application should be determined by the scope of the claims.

Claims

1. An adaptive time-window line differential protection method based on transient frequency band energy characteristics, characterized in that, include: Obtain the differential current at both ends of the protected line, and determine whether a fault has occurred based on the differential current; When a fault is detected, a transient analysis window is constructed. The differential current within the transient analysis window is decomposed into frequency bands to obtain low-frequency action energy and high-frequency braking energy. The ratio of the low-frequency action energy to the high-frequency braking energy is calculated to obtain the energy ratio. Based on the low-frequency action energy and the energy ratio, it is determined whether to execute the fault protection action within the zone; When it is determined that the fault protection action within the zone should not be executed, the length of the transient analysis time window is extended; The low-frequency action energy and the energy ratio are updated based on the extended transient analysis time window, and it is determined whether to execute the intra-zone fault protection action based on the updated low-frequency action energy and energy ratio.

2. The adaptive time-window line differential protection method based on transient frequency band energy characteristics according to claim 1, characterized in that, Methods for extending the length of the transient analysis window include: Preset thresholds for increment length and time window length; The transient analysis window has an initial length; When it is determined that no fault protection action will be performed within the zone, and the length of the transient analysis window is less than the window length threshold, the incremental length is increased based on the initial length to extend the transient analysis window.

3. The adaptive time-window line differential protection method based on transient frequency band energy characteristics according to claim 2, characterized in that, The method for extending the length of the transient analysis window further includes: When a fault is detected, the fault initiation time is determined based on the differential current; The transient analysis window starts at the fault initiation time and extends to the end point based on the incremental length.

4. The adaptive time-window line differential protection method based on transient frequency band energy characteristics according to claim 2, characterized in that, Also includes: If it is determined that no fault protection action within the zone will be performed, and the length of the transient analysis time window is extended to a value not less than the time window length threshold, then the fault is determined to be an external fault.

5. The adaptive time-window line differential protection method based on transient frequency band energy characteristics according to claim 2, characterized in that, Methods for determining whether to execute intra-zone fault protection actions include: Calculate the ratio of maximum low-frequency energy to maximum energy for various types of out-of-area faults within the transient analysis time window; A preset reliability coefficient is used; the maximum low-frequency energy is multiplied by the reliability coefficient to obtain the low-frequency energy setting value, and the maximum energy ratio is multiplied by the reliability coefficient to obtain the energy ratio setting value; Determine whether, within the transient analysis time window, the low-frequency energy is greater than the low-frequency energy setpoint, and whether the energy ratio is greater than the energy ratio setpoint; When it is determined that the low-frequency energy is greater than the low-frequency energy setting value and the energy ratio is greater than the energy ratio setting value, the fault is determined to be an intra-zone fault. When the fault is determined to be an intra-zone fault, intra-zone fault protection actions are executed.

6. The adaptive time-window line differential protection method based on transient frequency band energy characteristics according to claim 5, characterized in that, Also includes: Based on the initial length, incremental length, and time window length threshold, the length of all possible transient analysis time windows is determined; Calculate the low-frequency energy setpoint and the energy ratio setpoint within each transient analysis window; Based on the length of each transient analysis window and the low-frequency energy setting value and the energy ratio setting value within each transient analysis window, a setting value comparison table is constructed; When determining whether to execute the fault protection action within the zone, the low-frequency energy setting value and the energy ratio setting value corresponding to the length of the transient analysis window are called from the setting reference table according to the currently constructed transient analysis window.

7. The adaptive time-window line differential protection method based on transient frequency band energy characteristics according to claim 1, characterized in that, The method for frequency band decomposition of the differential current within the transient analysis window includes: The differential current within the transient analysis window is divided into frequency bands to obtain low-frequency action components and high-frequency braking components; The energy of the low-frequency action component within the transient analysis window is calculated to obtain the low-frequency action energy; the energy of the high-frequency braking component within the transient analysis window is calculated to obtain the high-frequency braking energy.

8. The adaptive time-window line differential protection method based on transient frequency band energy characteristics according to claim 7, characterized in that, The method for dividing the frequency band of the differential current within the transient analysis window includes: The differential current within the transient analysis window is decomposed using a complete set of empirical mode decomposition algorithms to obtain multi-order intrinsic mode function components and residual components; Calculate the zero-crossing rate of each order of intrinsic mode function components within the current transient analysis window, and obtain the equivalent frequency of each order of intrinsic mode function components based on the zero-crossing rate; A preset frequency boundary value is set; the intrinsic mode function components with an equivalent frequency not greater than the frequency boundary value are classified into low-frequency action components, the intrinsic mode function components with an equivalent frequency greater than the frequency boundary value are classified into high-frequency braking components, and the residual components are classified into low-frequency action components.

9. The adaptive time-window line differential protection method based on transient frequency band energy characteristics according to claim 1, characterized in that, Methods for determining whether a fault has occurred include: Preset current surge trigger value; Obtain the differential current at each sampling time; Calculate the difference between the differential current at the current sampling moment and the differential current at the corresponding sampling moment of the previous power frequency cycle, and calculate the magnitude of the difference to obtain the differential current abrupt change. Determine whether the differential current surge is greater than the current surge trigger value; if the differential current surge is greater than the current surge trigger value, then determine that a fault has occurred.

10. The adaptive time-window line differential protection method based on transient frequency band energy characteristics according to claim 1, characterized in that, Methods for determining whether a fault has occurred also include: Preset zero-sequence voltage mutation start value; Obtain the zero-sequence voltage at both ends of the protected line at each sampling time; Calculate the difference between the zero-sequence voltage at the current sampling time and the zero-sequence voltage at the corresponding sampling time of the previous power frequency cycle, and calculate the magnitude of the difference to obtain the zero-sequence voltage abrupt change. Determine whether the zero-sequence voltage mutation is greater than the zero-sequence voltage mutation start-up value; if the zero-sequence voltage mutation is greater than the zero-sequence voltage mutation start-up value, then determine that a fault has occurred.

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